When should I test?

Hi there, I have a question. My last period was on July 10th. I don't have regular cycles; I stopped the pill in February, and after that, the time between my periods was 4, 4, 4, 5, 5, 6 weeks. So, regular? No, not really. But here's my question: I had a little bit of bleeding on the 15th and 16th. It was very light and brownish. (It was so little that it didn't even show up in my underwear; I only noticed it when wiping for sure. I put a tampon in, and yes, there was a bit of brownish blood on it.) What do you think it was, and when should I test?

Best, Anno