Please help. Swelling in and pain behind both knees

 woke up Wednesday morning with a really sharp pain behind both my knees which hurt to bend. Behind both knees there was a swollen area which felt like they were full of fluid. There was no bruising and no real redness. I don't suffer from arthritis and now 3 days on my right leg has returned to normal but my left is still slightly swollen. 

Does anyone know what could've caused this? I haven't felt ill or had any other symptoms. 

I was drinking the night before along with boiling hot weather and a fair bit of walking but found it strange that behind my knees swelled on both legs. 

Please help! 

Thank you

To add Iv been told by a few people it could've been due to the hot weather but I'm not too sure

Sounds to me like a bakers cyst . I have had  them, sometimes they stay for months 

pam

Hi Pam,

Would they affect both knees at the same time? And like I say my right knee now feels a lot better where as my left is still a little sore

I am not a medical person, but I have had them in both knees . Felt almost like a golf ball ..usually a sign all is not well. I have just had my first knee replacement. 

Yeah, mine feel a bit like flat, puffy sort of swollen compared to a golf ball

They were difficult to feel unless I was standing with my legs straight rather than bent .