Hi Scarlett,
sorry to hear you have problems with your battery.
There has already been a discussion on this here.
First of all, using a bloated battery can be dangerous!
You should definitely contact Fairphone support by clicking on this link and tell them about your situation. Usually they react quite fast to this and send you a new battery. Probably this is a warranty case!
I don’t know if you have to send in the battery, but there is probably no need to send in the whole phone.
All the best