Ditto to what the others said---be your own best advocate. I was diagnosed at 30 so Julie is right, don't let them tell you that you are too young, However,......80% of lumps are benign, so don't write this off as cancer yet! Most likely it is benign, but you do need to get it checked. Don't take no for an answer. We don't mean to scare you, just remember we were the ones on the other side of that statistic.
When I found my lump, I went to the GYN first, just because it made sense. I then had to go to my PCP because I had an HMO, and any further tests or referrals had to come from her. I guess it would depend on your insurance (do you need the PCP to coordinate things?).
Good luck, keep us posted!