Pain in my leg & behind knee. PLEASE HELP :(

About three days ago after standing up from laying down in bed in a perfectly comfortable position (I wasn't sleeping, just was on the laptop) I began to adapt a pain behind my knee, like a slight ache. The next day, I found it increasingly harder to walk. I could fully balance on my leg, with no sign of leg weakness or anything, but the place behind my knee would really hurt and ache when trying to walk so I had a ridiculous limp all day. Now, the pain has subdued while walking slightly, but there is now slight pain on the back of my thigh and calf with some type of twitches on all three of these areas, kinda like something is "bubbling" under my skin. I've been doing thigh and calf stretches to see if it's to do with my muscle, but they don't seem to be helping and I get a weird sensation while doing the stretches, a slight ache, especially behind my knee area. There is currently no type of swelling or redness, no pins and needles or numbness down my leg or foot, and I feel completely fine everywhere else.

I have extremely bad anxiety problems, so I'm panicking so much thinking that I have deep vein thrombosis, a blood clot or some other serious illness. Already had one bad panic attack tonight over it. Nothing seems to help the symptoms and I just want to go to sleep :(

I rung up NHS direct but they said they are extremely busy and will have to ring me back in about 6-8 hours time. I don't know if that's a good thing or a bad thing, as if it was an emergency I'm sure they would have told me to go to A&E or ring an ambulance. So I'm just going to sit up and wait for their call back :\

If anyone has any advice, or has had similar problems in the past etc. then please comment on this thread.

Thank you in advance :)