Can't stretch knee beyond a certain point?

A week ago when I was walking up a hill I got to the top and I had a pain in my knee and I basically had to limp home, then it gradually got better. The same happened a few days latter and it happened yesterday. Yesterday being the worst. I was in bed when I moved my leg and I had a massive shooting pain come from my knee and I had a hot flush and felt that I was going to be sick. I was in enormous pain until it eventually got better throughout the day. So I went to the doctors this morning and they basically said nothing was wrong and that my knee is completely functional and completely fine and that I should take some aspirin next time. I don't know about anyone else but I think this is pretty shitty medical care, it happened 3 times so what if it happens again and 'nothing' is apparently wrong, I just take aspirin? The doctor said he has no idea what is wrong (and empied that I was having sex or something of the sort which I was not, I simply moved my leg). I could not stretch my knee beyond a certain point untill the next day and now the doctor says it is fine