I am currently 11 weeks pregnant for my first time. I worked out and lifted weights before I became pregnant, but I have not worked out since I found out I was pregnant. I decided to get back into the routine slowly and wanted to walk for about 20 minutes. I started to walk on the treadmill at a 3.2 level (very easy, still able to have full conversations). At about the 10th minute, I started to notice the inside of my legs, the fronts of my knees, and the back of my knees starting to get hot. Then all of of a sudden I became short of breath and dizzy. My knees were red from the front and back, hot, and itchy. There was no swelling present. Just so you know, I am 5'1" 120 pounds, so not overweight. What's going on?