How long after quitting smoking does your metabolism become normal?

I quit smoking in January, and I swear my metabolism slowed down a lot. I gained weight and it seems much harder to lose this weight than other times I've gained and lost. Is it something to do with my quitting smoking? I'm trying so hard and I feel like nothing is happening sometimes. Sometimes I feel good, but other times it's like I'm stuck.
I'm not eating to replace smoking. In fact, I'm eating less because I'm watching my diet.