I’m scared

Last month I bled dark blood for about four days but it was very light bleeding. It’s was irregular and unexpected for me to be bleeding in the middle of the month also, it was very dark blood idk... Now I’m worried that I might be pregnant bc I missed my normal cycle. I did take birth control for 3 or 4 days but stopped taking it bc I have stomach problems and it was making me nauseous after every meal. I have had vomiting but I’m certain it was my weak stomach.