This month i have lost 20 pounds,just eating healthy & exercise.i've been very constipate.today i strained for a bm and got a bad migraine.is this bad?

Water, water: In eating healthy and exercising you might have lost enough fluid to both make the loss quite high and the bowels constricted. Be careful to drink ~1/2 gal/day. For now, a gentle laxative might help - or better - prunes, warm lemon water, etc. Try not to strain, hopefully an OTC pain med will clear the headache. It's not likely migraine.