My girlfriend is sore after nearly every time we have sex and starts being sore after only about 10 or 15 minutes of sex. She says it is sore muscles, not skin so it's not about being properly lubricated. We have a considerably amount of foreplay focused on her and usually don't start penetration until she is literally pulling me on top of her. On top of this, she is definitely not the tightest girl I've been with. I'm 6 inches around so I'm also not huge. Any thoughts?