I passed out numerous times in the first trimester (usually while throwing up -- not pleasant!), and I often have to lay down with my legs up to avoid it. I have really low blood pressure, and passing out is just something I do, pregnant or not. If it's blood pressure related, drinking a TON of water can help. Also avoid showers that are too hot (the heat draws the blood to your extremities and away from your head).
My OB isn't concerned. She actually congratulates me on my low bp at each appointment. I realize high bp is a problem, but super low bp isn't a walk in the park!
Just remember to get those feet above your head!
Oh, and as far as feeling that way while having a BM... If you were, um, pushing/straining, it can cause something called a vasovagal response that will lead to feeling like you'll pass out.