Try to figure out where these feelings are coming from. Were your last births positive? Are you reading too many of our negative stories to relax? Can you pinpoint exactly what it is - is it something in the way the baby is moving/not moving, positioned, your hormones/behaviour.... that is different than the last two pregnancies? If it is something specific, perhaps you might find out if there is a simple test (urin, blood...) than can be checked to "give you an answer" and therefore reduce your fears. (OK, I might get a bit flammed for that comment, but IMO if it will make you feel more confident or relaxed, then not all tests are evil.)
It might also be helpful to remember all pregnancies and births are so different. Also talk to DH or a friend, see if you can find out what is going on. I'd love to say relax, take it easy, but sometimes that is easier said than done.