I had the same thing when i was about 20 weeks. I went to the doctors and they did blood tests every second day to make sure everything was okay and they couldnt find any results. It lessned within about 3 weeks and just went normal. Hope its the same for you. Best of luck x
It is probably iron deficiency/anemia. Tell your doctor when you go in next so they can check. If it is, they'll put you on an iron supplement. Til then, eat foods high in iron! Red meat, beans, green veggies(spinach is really good for iron) fruit, etc.