What can make your muscles cramp up, like a charley horse?

Many reasons. Poor blood circulation in the legs overexertion of the calf muscles while exercising insufficient stretching before exercise exercising in the heat muscle fatigue dehydration magnesium and/or potassium deficiency calcium deficiency in pregnant women pinched nerve side effect of some drugs lasix, statin medications donepezil Neostigmine Procardia (nifedipine) proventil and asthma medications.