I had the same at around 7 weeks with DS. They weren't sure what is was from but told me to rest up and see how I went.

I can see you have 3 others kiddies to run around after but maybe try and rest up as much as you can. There's not a lot the Drs can do to "help" you besides an u/s.
Give them a call if it will put you at ease though hun. Thinking of you