Viagra doesn't actually help with feeling arousal, only the mechanics of blood flow. There is talk about releasing a female version if Viagra but if the reason you're getting physically aroused is because of lack of desire, it probably won't help much, sorry.
The vast majority of libido problems are psychological in nature, so you would be unusual if that's not the case, which is probably why you aren't getting taken seriously by the health professionals. Even if you don't think it's psychological it may be but not easy to work out, which is worth thinking about, but I'm going to take you at face value and believe that your problems are physical.
You seem pretty adamant that it's linked to childbirth and the urge to have sex is in part controlled by hormones, so if you've ruled out all other causes, I'd say your problem will almost certainly come down to your hormone balance.
Are you breast-feeding? That can have quite an effect. Are you taking any other medication? Are you using any form of hormonal contraception? If so, consider changing or stopping altogether and using barrier methods.
Do you have any other hormone-related symptoms? E.g unwanted hair growth/changes, more painful/heavier periods than normal, more headaches, cycle-related anxiety, etc.
You could go back to the Dr and ask for hormone tests to be run. In some cases your Doctor will report only a minor change and tell you it's well within normal range and not significant enough for the Dr to be concerned about, let alone prescribe treatment. However, even in this range it can be enough to cause a loss of libido, the same as those suffering with underactive thyroids often experience awful symptoms before their levels are measured sufficiently low to warrant treatment.
Unfortunately, if you fall in this bracket, the only thing you can do is really work on your physical health and hope you're one of the lucky ones for whom being in optimum health is enough for your hormones to reset to properly functioning levels. Stop alcohol, eat an optimum diet and exercise regularly. You can also try to boost your oestrogen and testosterone levels by eating foods believed to boost these levels or buying supplements claiming to do the same.
Good luck.