I previously had hyperthyroidism, but now i'm gaining weight rapidly despite consistency, is it possible that i now have the opposite thyroid issue?

Since I'm not sure: what you mean by "despite consistency", I cannot answer this question directly. If you have had a radiation therapy or medication to help ablate the thyroid, to reduce hormone levels down to normal, or a surgery to remove part of the thyroid, then you need to have your levels checked regularly (usually every 3 months). Any of the above can push you towards low enough thyroid to give symptoms.