Is the cause of my headaches linked to my glasses?

About a month ago I got new glasses and no they're not very strong (-2.0 in both eyes) my optician told me to wear them all the time but I don't. If I wear them for a little while I get a horrible headache and so I take them off and the next day it's fine. This only happens when I wear them and I need them if I'm going out or watching television etc. I can see clearly through them so I assume they're the right strength. Any help?