When to test?

So I'm always irregular when it comes to my periods some times I get them every 3 months or even up to 4 months apart so idk when to actually take a pregnancy test My last period was August 5th (first day of period) and I took a test on the 20th of September it looks positive at first then negative so I thought it might be to early to tell so I waited till today and this is what I got I say negative but maybe you lady's can help or give me a little advice