Had blood drawn a week ago which really hurt. now i have some sorta muscle pain or cramps and a rash came out on my forearm. i'm scared its thrombosis?

Depends : Injury of any tissue can hurt for some time. A little black and blue is also possible. If it has swollen up, red, warm, fever- it could be sign of inflammation or infection around the area. Small superficial thrombophlebitis is local inflammation, gets better with warm pads elevation - are not as serious as deep vein thrombosis. If it does not better, might be a good idea to see your doctor.