I use to tan pretty quick and easy but now for some reason I can't get a good tan What has happened with my body to cause this?

I don't think its you. the change of the season usually relates, if its winter, then the UV rays aren't as strong, but if its summer you have a better chance of getting a tan. Also, when you go out in the sun to tan use sun screen on you skin.

If it's not a seasonal thing, it could be a change in body chemistry or the result of taking certain medications. If you are taking medication, check with your doctor or pharmacist to see if that is the cause.