Trouble getting wet?

I haven't been able to get very wet the last couple of weeks. In the moment I'm turned on and wanna have sex but it doesn't seem like my vaginas on the same page. It's always dry and it sucks because it's been hurting during intercourse because there's not enough lubricant. I know you can use lube but I just wanna know why. I'm on birth control but have been on the same pill for like 2 years and this is the first time I've had a problem.