Extreme fatigue could be the meds you're on (fatigue is/could be your side effect to it), it could be low iron/anemia, your adrenals could be low (you state you have had more than one pregnancy), your thyroid could be low. The little one is taking a lot out of your body while he/she is growing; it wouldn't be unnatural for your body to be low or depleted in something. And if your body is already low/depleted in something from your previous two pregnancies--well, I'd seriously bet your body is clamoring for something it doesn't have right now or that's rapidly depleting (hormones, vitamins, minerals, amino acids, enzymes, etc.)
I'd mention it to your doctor and have your doctor do some intense lab work to see if you can pinpoint the problem.