Possible DVT - No insurance

Hello, hello. Hopefully I can get some answers via this website. I am a 26 year old female who is slighty over weight and a smoker. 7 days ago I started to have throbbing pain behind my left knee. The area appeared slightly swollen & bruised/discolored in the crease. By day 3 the throbbing, crampy pain had spread to my thigh and rear of my calf. I'm am without work right now, so I spend most of my days sitting on the couch. There has been no injury, even minor, to my leg or knee. That being said, with worsening pain I suspected DVT so I went to the emergeny room. The very cocky doctor said since there was no heat of "major" swelling, I was psyching myself out. I pushed for and received an ultrasound which came back negative for a clot. They did not run a D-Dimer test. I was sent home diagnosed with a pulled muscle (I don't know how I could possibly have pulled one) and I was told it should heal on it's own in a day or two. That was 4 days ago and the pain keeps getting worse all over my leg. It's throbbing, crampy and everywhere from thigh to foot, although it's worse behind the knee. Swelling has increased a tiny bit and it is now painful to walk. I am also now experiencing "pins & needles" sensations all over my leg. I am terrified the ultrasound may have missed a clot since the doctor & ultrasound tech didn't seem to take me very seriously. I do not have insurance and certainly cannot afford another trip to the ER - I also do not qualify for any state health assistance. My questions are as follows . . .
1)How possible is it that an ultrasound could have missed a clot in my leg?
2)Is there any way to treat a suspected clot on my own - ie asprin in a certain doseage?
2)Could a pulled muscle realy cause full leg pain like this?

Thank you in advance!
Kari