I take 100mcg of levothyroxine daily. if i forget to take it for a day or two, will it cause me to feel bad? should i double up a day or 2 to catchup?

No and yes: Hi. L-thyroxine is a great drug. If you miss a day or two, your body will never it. Be as compliant as you can be, of course. If you miss one day, take 2 the next day, or if you miss 2 days, take 3 the next - that's fine. Due to the long half-life of l-thyroxine in the blood, your body will never feel variations in dose over a day or two. It's best to take catch up doses so your next labs are true