Twice since the winter I couldn't get it up when the gf and I tried to have sex. The gf has been a bitch a lot this past month or two, and she has been bringing this up a lot, and sunday past, it happened the second time. She says that I should get my pecker checked out, but I keep telling her it was because I was dead tired both times. She can't understand that. Is it tiredness a cause for no erection?