oh my god does thanks a lot really sound ominous to native speakers D: I’ve also been saying it forever!!!!!!!!!!! nooooooo
I think it depends on your culture (like it sounds like that to me, being from the UK, but it might be totally normal in the US). Also, it only freaks me out by text, if someone says it out loud and I can hear their tone, it doesn't even register! Definitely don't worry about it, we all have our personal "dialect" in a language, the words we love and the words we hate :)
(if you want alternatives though, "thanks so much" reads much more friendly to me over text!")