I haven't read anyone else's reply but I have/had the same problem. Except I am a guy but I am 1) at least 20-30lbs lighter than everyone else in my class and 2) Im atleast 5 years younger.
So I am familiar with bruises on my forearms especially. All I can say is that I stopped bruising once I got used to taking hits and I improved my holding technique.
Make sure that you are "pushing" into the strike, and than "shrinking/absorbing" it. If you are a wall than its gonna hurt.
You have to move into the strike as its thrown, and once it makes contact, move back with it. This way you are not just taking it straight on. But also... if you are bruising easily, it probably means that you lack conditioning to be able to spar and such. So think of it as conditioning your body to be stronger. That's why I always partner up with someone bigger:
1) I can go full out without worrying about hurting the guy
2) I practice taking strong hits so that when I fight someone my size, they can't hurt me.
Hope I helped