Hello, I would be happy to help you with your question.
While it is extremely unlikely that you would have conceived in mid to late March and not have a positive test by now. The negative test strongly suggests that you are not pregnant. This, though, should be re-checked in 1-2 weeks. The cycle irregularity might be related to irregularity in terms of when you ovulate. Birth control pills will regular your cycle and decrease the chance that you will end up with an unplanned pregnancy
I hope that this helps. If you have more questions, please click on "ASK ME A QUESTION" link above to directly ask me a question!