irregular periods after amenorrhea

Helloo, my period stopped for about 15 months due to being underweight/restrictive eating. I started menstruating again about a year ago, but my periods are still very irregular, is this normal? Will it take time for them to regulate again? Prior to amenorrhea my periods were always regular.

Not sure if this is relevant but in the last year my BMI has fluctuated between 18.5 and 20, for the most part it has been around 19. 

Gosh.  It could be alot of things.  It is possible that your body doesn't like being at 18 - 19 BMI.  Especially if you have low body fat. A 19 might be to low for you, especially if you have to keep eating under your maintenance calories to maintain it.  Have you been eating your fats?  I'm talking things like olive oil, nuts and seeds, avocados, etc.  Sometimes a diet too low in fats could cause the period to go wonky as well.

If it's not any of that, I'd say go back to the doctor.  There might be other issues that we have no way of telling online.

How old are you?

Honestly, when periods return they can be irregular for 6 months to a year. Usually, as hormones stabilize the period will also stabilize. Most women, REGARDLESS of 18.5 being classified as "healthy", require a BMI of 20 to maintain proper body fat percentage for hormonal (reproductive and thyroid) balance. If you can keep your BMI at 20 or just over, you may find they stabilize more rapidly.

If you are concerned, contact you OB-GYN; I do not, however, recommend using BC to "regulate" you at this point as you want to make sure you are healed - "fake" periods may make regularity equally difficult when you stop suddenly. A hormone panel taken by your gynecologist may help indicate a problem. If all is well, then keep your BMI at 20 or just over, don't overexercise, get enough quality fat, and give it the 6-12 months after menstruation resumed.

I'm 18. I admit I don't usually eat much fat at all but I have tried to up my intake in the last few months, just a bit stuck in my ways with my staple foods and such.

I did think my periods were becoming more regular in the last 3-4 months or so, but this month I'm really really late again, it's so frustrating. I have recently lost a tiny bit of weight (like 7lbs) because I was ill so I dunno if that has anything to do with it - I didn't think such a small loss would effect me much though. 

Thanks for the replies though, maybe my body isn't very happy at this weight.