I am 21 years old and I can't get hard like I used to

Well I don't know what's wrong with me I use to get hard a lot then one day I couldnt get hard at all it really upsets me.. I went to a urologist and got some test done but he couldn't see anything wrong... It's like when I'm in the mode my penis gets hard then gets soft like right away when it first started to happen but now it feels like its getting better but still gose limp.. What's wrong with me.. Bh