2 days post period. Intense dull deep achy type pain below belly button? Serious or not?

Likely not. If no fever, nausea, vomiting probably ok. It could be your bladder if peeing a lot. If persistant get seen but try 2 aleve (naproxen) 2x per day. If its a new type pain i would put my money on bladder but likely not life threatening. Go in if it worsens though.