I have a healthy sexual relationship with my girlfriend and we have sex almost every week. My sexual history has been somewhat unorthodox as I have never masturbated with hand but rather with my thighs. 

When I started having sex with my girlfriend I always used to stay hard for long but never used to be able to cum inside her and had to climax masturbating. But a few weeks ago I was able to climax inside her in a normal way and after that everything has been going wrong. Now I am unable to get hard when I get naked with her. A few days ago we were dry humping and I was hard for quite a bit but last night when we tried to have sex again I got soft immediately as soon as we took off our clothes. It's getting frustrating everyday and would be really grateful if there is someone out there who can help me.