Lots of bloodwork..

Try to keep this short.. I'm 27, have two kids and I've ALWAYS had regular periods, on the heavier side, ive never had bleeding in between periods, nothing out of the ordinary. My husband and I only started trying to conceive our third a few months ago, but as soon as we started trying I have had two very light periods, sore breasts, vivid dreams and cramping, but only negative pregnancy tests. I went to my doctor's today and she said we could do a blood pregnancy test, as well as a bunch of other tests. Has anyone had any experience with this? what are the odds of being pregnant? what else could this be? I just find it so odd that this is happening as soon as we started trying. I'll add too that both pregnancies with my other 2 were totally normal, no bleeding, got postive tests at home.