Can I lose strength just because I'm pregnant?

So, I'm currently 16 weeks pregnant with my first child.

When I do strength training these days, I often find myself much weaker than I was before pregnancy, especially in my upper body.

I can no longer lift the weights I used to lift. I can hardly do full pushups. And I don't understand why.

I've been eating the same, and working out the same (except for a few minor modifications here and there) as in my pre-pregnancy routine, and my belly hasn't even grown much yet.

Really, I can't think of a reason why I've already lost so much strength :(

Do other moms experience similar things, or should I be worried?