Mood changes are certainly a possible side effect of birth control pills. If her birth control is reducing her sex drive, she could be feeling less romantic towards you. If she has concerns about that, then she may want to switch to a different pill to see if things improve.
However, if her feeling differently about you is the only thing that has changed, it’s also possible that this has nothing to do with the birth control. You said you have already broken up, so ultimately this is her decision. Mood changes, unless extreme, don’t stop someone from having the ability to make a rational decision.
Relationship endings are hard, and we want to find something to blame. That’s normal and can help us cope with the loss. At the end of the day, however, the relationship is over, and you have to respect her choice. Be kind to yourself, stay active, and surround yourself with friends.