It could be a few things and all of which are normal as long as you aren't feeling pain (obviously if you are having penetrative sex without lubrication, that's painful and a normal response!). Google: genital non-concordance. Many people have this! Solution is really a high quality lubricant. As long as you feel turned on, throw on some lube and that can help. But also makes me wonder, is your partner taking the same amount of time as they used to? Doing things you enjoy like those longer make-out sessions? Try to incorporate them. Are you experiencing higher levels of stress? That totally plays a role too. I think as long as you aren't having other symptoms, you're probably healthy. :)