I have major ear issues all my life, and this time of year it is the sinuses, as they are very sensitive. Fluid builds up and such, and when you lift like you already know the blood pressure builds-up in the cranium, so most likely with the flow of blood in the tiny vessels in the ear drum area, aside with fluid and such, seems very likely to be the cause. Avoid water getting in from the shower for the next few days, and just watch your stress/straining levels when lifting, and of course breathe.