When To Take the Test

I'm very late on my period! I was thinking it was stress that was pushing it back since the last month or two have just been horrible for me. but it has never been this late before. my average cycle is 29 days, that is pre depo shots. my first cycle after I stopped taking the shots was about 30 days. I am now on day 40. all symptoms are there, just a no show of my period. I've already taken 2 pregnancy tests and they were negative! was it too early for the test to be accurate?