Why does it seem my overbite is getting worse? I grind at night

Growth. Jaw develops for years sometimes after you have finished growing. Grinding can affect your TMJ joint making this worse.
It probably is. Bones that support teeth change constantly. Overbite will worsen if you have an untreated malocclusion, if you had treatment and didn't wear retainers exactly as directed, or if you grind your teeth and don't wear a night guard. Before it gets out of hand go see a qualified orthodontic specialist. You'll be very glad that you did.