During pregnancy, pressure on the inferior vena cava (the main vein that carries blood from the legs) from the uterus when women lie in certain positions
Increased abdominal pressure when straining to move bowels or pass urine or when lifting heavy weights
Blood vessels become dilated (widened)
Alcohol
Some allergic reactions
Some antidepressants, such as amitriptyline
Antihypertensive drugs that dilate blood vessels (such as calcium channel blockers, angiotensin-converting enzyme inhibitors, and angiotensin II receptor blockers)