Weight question, all answers help!

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Ok so I'm 5' 4" and pre-pregnancy weight is 120lbs. I'm extremely active, I have a 3 year old and a 2 year old (never have time to be lazy lol) but I'm 18 weeks and I only weigh 122, my ob is a little concerned about it...this is my 3rd pregnancy and I never experienced this with my other 2 pregnancies and I'm getting very worried..help?