What causes bulging veins in the thigh that leaves red bruises?

Varicose veins: This is usually due to varicose veins. The veins are thinned out and sometimes you get spider veins. Also over time the skinn is thinned out. That makes you more prone to bruising, especially if you are using Aspirin. Keep the legs elevated and wear ted hose stockings. If problem persists see your doctor to do a venous ultrasound to check for venous reflux and treat accordingly.