Surfing Waves

Hi there. Right i have been surfing for about a year or so, and i am 20 now. I was just wondering if anyone could help me with my problem. i think i am now to heavy for my surfboard but not sure how to tell. The first sign that i thought i saw was that when after i catch a wave and i am left standing on the board for that short few seconds before diving it, is that my board sinks with me standing on it, only when the waves has left though not when i am catching it. (do not know the height or weight of my board by hand)

It's perfectly ok to have your board start sinking if you are not on the wave. It's if it sinks when you are up and riding that there's a problem
I'm assuming it's a shortboard? How far does the waterline come up to on you when you are sitting on it?