Same thing happened to me, Winnie. It took about two months for my body to adjust to eating healthy foods and an increase in fiber.
I was so concerned, I saw a GI specialist, since I had never had this problem before. We did some rather disgusting, but painless, testing to rule out anything major, but here's what I've done to get somewhat regular again:
Increase your water intake. Fiber bulks up your BMs, so you need water to "move it along". If you don't get enough water, you'll end up constipated.
Eat an apple every day with the skin. I don't know the science behind this, but my family doc and a friend recommended this, and it really seems to work.
Daily walking, even if it's only 15 to 30 minutes. Seems to wake things up in the tummy.
Make sure you're getting some good fat in your diet. I use olive oil, about two teaspoons, plus a little fat here and there from just eating healthy foods. If I notice it's been a day or two since a BM or very small BMs for a couple days, I'll put two teaspoons of olive oil in a little dish with some herbs and dredge a piece or two of crusty bread in it with my dinner.
Try and increase your fiber intake gradually. I went from about five grams per day to around 30 over the course of a day or two. This is what caused me to go to the doctor. He suggested I increase fiber by about five grams per week, along with extra water, and allow my body to adjust.
Oops..almost forgot. Freshly ground flaxseed helped me, too. One tablespoon ground up on your morning cereal or oatmeal. Some put it in their yogurt or sprinkle on top of a salad, too. It supposedly has other good health benefits, too, but I definitely noticed a big difference using that for a couple days. I find I can't use it anymore, or else I get the opposite effect now, but it really helped in the beginning. I found the taste very unusual. I had to experiment to see what I could tolerate it on.
Thanks for bringing this up. Not a wonderful topic to discuss, but I think this does happen to a lot of folks when working on their eating plans, especially in the beginning.