dude it's the miscarriage. and the birth control may not be helping, but it is not the cause.
give her some time, talk to her about it, issues like this usually don't just go away you have to work through them... sure you could get a quick fix w/drugs but they probably won't work entirely and it would just make this problem worse and bury it deeper. would you advise someone to take aas if their diet and training weren't in check? no you'd tell them to get their shit straight then when they had the basics worked out if they still weren't happy to use drugs.
sex and procreation are directly linked... it's funny how modern man separates them and deals with sex as if it has nothing to do with procreating. sex without the possibility of a child may seem pointless to her now and in her despair she has given up on it, this is just one possible scenario she's dealing with...
so yeah dude step up and be there for her but let her work it out at the same time... miscarriages/unsuccessful pregnancies are very painful to women and can cause deep psychological trauma
...i just read that she's your GF and not your wife... were you the baby's father? if not you may need to distance yourself from the situation because she may be hanging on to whoever the baby's father is and not a situation you need to be in. also did she go on birth control before or after you started dating? b/c that would be a warning sign to me as well... but i could be paranoid.