why doesnt my penis get hard?!

well ive been wit my gf for almost 3 months and we began having since before we started dating, its been like 4 months wit sexual activiy. at the beggining , i didnt have any problem at all , my penis would just get hard , very hard within less than 30 secons just by kissing her or touching but now , since 3 weeks ago , my dick wont work , i mean sometimes it just wont get erect and when its getting hard then i think of my problem i guess and i wont get erect, and the more i think about the more i get frustrated, however , if i dont think about it , then everything goes good , i get my penis hard easily but if i think about this , that my dick will shrunk , then that happen , its pretty embarrased. is it because of stress?? and because i think about that while having sex?? by the way , 3 days ago i had a good sex and i did not think of my penis and everything went good.. any tips or advice