This is a bit of a flying post as I'm in a bit of a rush so apologies for the rather vague laymen's terms. I'm not a doctor but I have had a diastasis recti (although not a hernia) so I know a little about the subject.
Basically, everyone's rectus abdominus separates during pregnancy. This is a sheath of muscles that meet in the centre of your abdomen and, if you're six-packed up, form the vertical line down the centre. During pregnancy the muscles shift to allow room for the baby in the growing uterus. After birth, these muscles slowly move back of their own accord (usually within six weeks of a vaginal delivery and around 12 weeks after a caesarean) and should come together within about two fingers width. Your GP should check for this at your 6 week post-birth check and it usually involves doing a dignified sit-up on your GP's examination couch.
Sometimes, however, the muscles tear, especially if separation happens quickly (i.e. twin pregnancy or large baby, etc.) and so the rectus abdominus doesn't come together fully and you get a wider separation that doesn't close. This is called a diastasis recti and it can be as wide as 8 fingers or so in width. Because of the lack of muscle closure and support it can cause hernias, as in your case.It also means that you can have a very bulgy, pouchy stomach as there isn't the abdominal support or casing - hence the five month pregnancy look.
It IS one of those things in the sense that there isn't really anything you can do to stop it happening, but you can certainly repair it and I'm really cross with your GP that he/she has effectively dismissed your concerns on this. You have every right to get this sorted. Aside from the cosmetic issue, which I think is hugely important (it can be hugely knocking to your self-esteem after birth) it can also cause all sorts of back pain and pelvic issues (future issues with continence, etc.) as you don't have the muscular support that you should. These muscles are hugely important in supporting the back and stomach and help with posture. They also have an added influence on other abdominal muscles too, which often have to work harder to compensate for the lack of support in other areas.
Go back to your GP and ask to be referred to a specialist. There ARE lots of things that can be done to resolve the issue - physio to help you close the gap in your abdominal and also hernia operations should this be necessary. The resources are there and you may have to nag and push for them, but you have a right to get this sorted.
In the meantime, I'd really recommned googling the Tupler Technique as this is fantastic for helping with diastasis recti. It's a series of focussed abdominal exercises (very small but intense exercises that you can do in five minute bursts sitting on a chair, etc.) that would really help. It will take time as the muscles move back very slowly, but it does work. My separation is now closed. The only thing I would say is to avoid all abdominal exercises as these can often 'fix' the separation. The temptation can be to do lots of crunches and obliques in the hope that this will strenghten things but these actually aren't the exercises you need.
Equally, and I now it's not always possible with time and finances, but it might be worth looking into some pilates classes if you can. A group class probably isn't much use for you at the moment as you need very specific, focussed exercises, but a lot of pilates instructors are excellent at helping with this sort of thing.
Go back and see your GP or, if you really can't face him or her then see another one in your practice - you're entitled to. I hate the idea that, post-birth, we're all expected to deal with the fall out of childbirth and pregnancy with no more than a shrug and an acceptance that it's just "one of those things". It's not - there are things that can be done and things that should be done - both for the long-term health of your back and also your self-esteem.
Hope this helps and sorry for the garbled rush - I hope I haven't missed anything out. Kx
P.S. If you get time in the meantime, I can really recommend the following simple exercise. Go on all fours on the ground (knees under hips, hands under shoulders). Take a deep breath in and, as you breathe out, imagine pulling your belly button up into your spine. Breathe in and hold and then, as you exhale, allow your stomach to gently drop. Repeat 10 times. You can also do this and hold your belly button to your spine for ten seconds before you exhale. It's a very gentle exercise but it's fantastic for helping to pull the rectus abdominus together. I used to do this three or four times a day - morning and before I went to bed and then whenever my twins slept and it really helped. Gave me a bit of a (much-needed) boost too in that I felt I was doing something postive.