bigintn:

Try to find out first what your gf's surgery was for. You don't really know if YOU had anything to do with her prob. The question to her (from her doc) about sex and whether it "hurt" didn't necessarily indict---YOU!

Second, go easy with her, and go easy on yourself.

If this relationship is important to both of you, you can work through this. I don't know your size and I don't need to know. The foreplay is certainly encouraged if you are big. (In fact, I advocate foreplay--period.)
I can't help but believe your gf (like all women I would think) appreciate tenderness, affection, and some "warming up" for the big moment---before the act begins. And, lubrication is always a good idea to help things "go more smoothly" You may have to take your time in entering...build up to full penetration (as in over several different sessions!) following her surgery or, especially if you are larger in size. This is not merely showing discretion or empathy for her--it's more like common sense. If you are large, there's going to have to be a period of adaptation for her body to adjust to your size.

Again, if both of you want to continue...you'll find a way to work all this out.