Tingling, Numbness, and Pain...Oh My!

I have not changed anything about my bike since I got it fitted a couple of monts ago. Well, I changed the tires, but nothing about the fit. But for the last several weeks, especially this week, my rear is not happy with me while it's on the bike. It's not so much pain that is the problem. I feel like I'm compressing something below my ischial tuberosities. Like at the top of the thigh, just under the curves of my buttocks...very top of the hamstring. It's tingling, pins and needles type feeling. Not so much pain, but very uncomfortable. I still sit on the seat exactly the way I always have (at least as far as I know). I don't know why this just all of a sudden started happening. I'm doing a lot more hills but would that cause this feeling? Anyone have any thoughts about this?
Thanks
Gray

Have you increased your mileage? If so, how fast? Most experts recommend only increasing about 10% per week. Most new riders tend to increase a lot faster than that if they're enjoying riding. If you start with 5 miles, a 10% increase is only 5.5 miles, not much. I'd be more likely to do 5 miles, then 10 miles, a 100% increase! Sometimes it just takes cutting back on mileage a bit to get used to a new bike/position. Other times there's a saddle or fit issue. Unfortunately, this is very hard to diagnose over the internet!