I think this is a common thing. I've been running for several years and I'm almost 10 weeks pregnant. My pace has slowed down significantly since I found out I'm pregnant. Listen to your body. If you're tired, walk, if you're ok, jog.

I noticed similar changes first with weight training. I can't do as many reps per set--if I try to push it I feel light-headed and overheated. I also feel that with jogging.

I was an avid runner pre-pregnancy and ran my first Full a few months before becoming pregnant, I was actually planning to run another Full at 16 weeks, but dropped down to the Half instead, just to be on the safe side. I felt great running the Half and ran it in a pretty good time at around 1:50 - I did notice my stamina wasn't quite the same and I tired easily. I continued running roughly 25miles/week up until I was around 25 weeks or so which is when I experienced a really bad pain in my left groin/hip area. It hurt so bad I could barely pick my left leg up to put my pants on for work. The doctor said I more than likely pulled something, or since my tendons are becoming loose and pelvis is shifting to get ready for the baby, my left hip could've popped out of alignment, causing a pain in my upper left inner thigh/pelvis area. Needless to say, I'm still experiencing upper inner thigh tendon pain and pelvis pain so I'm taking it easy in regards to running. Slowly ran a 5k last weekend when I was almost 30 weeks, finished in 29:40, which is a lot slower than what I'm used to, but I didn't want to push it too hard with my left groin/upper tendon still bothering me. This week I'm back at doing the elliptical or group cycle and upper body weights 5x/week, and a tiny bit of jogging on the TM if my pelvis/tendon feels better. Just take it easy and take it day by day. It makes me feel good to get some type of physical exercise in at least 5x/week, even if it's not running, the elliptical and group ride is a great way to break a sweat, without pushing your limits too hard.