Maybe you're just getting more muscles :]
as for stomach aches, it could be a number of things.
Its not just a straight up symptom for pregnancy...
If you could really call it one at all.
If you're worried... Wait until you've missed a period & test.
I, personally, worried so much that I was imagining symptoms.
But i'm not pregnant! So just try not to worry, though
i know its hard.<3