It can take 6 months to get the shot fully out of your body. Since your periods were non existant on the shot, this is probably because of that just rolling over still.
I'm not sure how long your periods will be irregular for. It really depends on how fast your body gets rid of the shot and how your body reacts to your new birth control.
Some women on ortho tri cyclen lo, or really any birth control pill, may get a really light period and/or skip it as well because the dose of hormones in this pill is so low. I would wait it out a few packs until your body can fully get over the shot and if you are still concerned, talk to your doctor.