hjscho - sounds like you urgently need to see a paed. It can take a GP absolutely ages to get you referred if they don't think its urgent. I hate to say it, but exaggerate like mad to get your referal asap. Sometimes you need to make the situation look much worse than it is to get a referral quickly.
I agree that a 20 week old BF baby should look quite chubby. BF babies gain loads of weight in the first 3 months. My DS looked very chubby at that age. He began to slim down after 6 months, which is very normal for an excl. BF baby. Kellymom.com has an excellent explantation for this.
Where do you live ? If you can afford a private appointmnet with a paed. I can refer you to one in North London. He isn't a friend or anything so this isn't advertising. I have never had to use him (thank goodness) but when my DS was born I asked a lady at the NCT if she could give me a no. of a private paed. for emergencies because I know how slow the NHS can be. I still have his no. She said this guy had saved her son's life, so I reckon he must be good !
I really sympathise with your situation. A friend of mine had a very thin looking baby throughtout until he was about 5.5 months old. He looked like skin and bone. She was breastfeeding, but used a dummy very frequently from the day he was born and he also seemed to have reflux as he threw up practically after each feed. Both of these things can lead to low weight.
I suggested she take him to a paed., but she didn't. She ended up giving up BF and switching to formula (on the advice of her HV) without getting her baby checked to see WHY he wasn't gaining. I was really sad about this because it may have been remedied without the need to give up BF.
I am really glad you have raised your concerns here because you will get lots of good advice !
Good luck.