I fell preg on the pill - and had a one day medium period. Didn't think too much, just thought it was light. I went on to take the next 3 weeks of the packet but was starting to feel off - then felt preg and did a HPT. I ended up m/c though - and it can be harmful to take the pill whilst preg so if in doubt - yeah, use something else and do a HPT!

Good luck
Sam