I Have Been Having Bit Of Pain Pan On My

i have been having bit of pain pan on my left leg and it would sometimes throb for no reason. it is mild but sometimes it occurs on and off even if i am exercising. i can't really describe the feeling. i check to make sure there isn't anything there...there's no bruising or anything.
today as i bent down for a short time, i felt it hurt a bit more when i got up. just now, i saw a small red spot about the size of a thumb tack which wasn't there this morning. i don't know what it is. could it be something more serious?