Maybe, rather then wearing bikini bottoms, you should have your bikini bottoms on underneath a pair of swimmer shorts, and still have your bikini top on.
Lots of girls do this, and it wont make your bulge as noticeable.
FYI: While you think of it as a tumor, (I did myself) that makes you build up even more of a resentment and hatred for it to the point that you might even attempt self harm in that area (I know it got to that stage for me) that is until I saw my Psychiatrist who actually blatantly laid it all out to me. That thing between your legs is not a male bit, nor is it a tumor or a growth, it is a vagina that is out of it's natural shape. While you hate it now, when it is in it's natural shape, you will love it. When I start to get anxious about it, or disgusted, or start to think about attacking that area, I stop, and repeat to myself, over and over, that it is a vagina, it's just not in it's right shape. Only 10.5 more months until it is.