Ive been drinking a lot of water to lose weight so why did that give me a headache? any idea?

Head aches: Not sure if the excess water intake caused you head ache or maybe something you stopped eating/drinking. For example, sometime, when people diet and cut out soda with caffeine, they get head aches. Caffeine may not only cause headaches, but can also be a treatment for them etc. So play detective and slowly decrease food groups as opposed to stopping them abruptly. See doc if the HA continues.