Lose my period when I work out?

This is probably something that you want to see your OGBYN or doctor about. I was thinking that you might be eating too little, but 1800 calories is pretty good.

This! Bring your food diary and exercise log with you.

I agree with the pp's; I would guess there is another underlying cause. Definitely check with your ob/gyn.

Are you tracking your iron intake? Being severly anemic from poor iron intake can cause loss of periods and even an 1800 cal diet can be low iron if you aren't eating any iron rich foods. Working out puts extra strain on your body which is going to up your micronutrient requiremnts and aggrevate any malnutrition you may have from your diet.

If you are logging your food MFP tracks your iron intake. Go to the reports tab and there is a drop down menu and iron is one of the choices. Check it out.

Paleness, lethargy and dark circles under your eyes are other symptoms of anemia.

Definitely have yourself checked out medically, since it's probably not your diet/exercise or weight, and you say your body fat % is probably not that low.
How long has it stopped for? I once went 62 days between periods for no discernible reason - there was no chance I was pregnant and it definitely wasn't exercise or diet related. Bodies are just weird that way.

Definitely see a doctor.

In the meantime, I'd try to increase your intake a bit more. I actually disagree with some comments here that say that you're eating enough - or at least that you're definitely eating enough! Yes, 1800 sounds a lot, (especially when you hear so many people getting by on a lot less), but it sounds like you're doing a lot of exercise for that intake.

I have been reading a lot about this topic lately and it seems that firstly, women's bodies vary in terms of how much bodyfat they need to menstruate, but also that too large of a calorie deficit (whether through intake, exercise, or both) can cause problems, even if your bodyfat is high enough. And stress definitely plays a part.

I would consider upping your calories a little, and maybe cutting the exercise down a little too. Find the sweet spot where you can still lose weight (slowly) and get fit, but you can get your period back. Personally, if it was me, I'd actually try to stop losing until you got it back, and then try again, with a very small deficit. It hasn't been a long time yet, but given that you've had problems before, I wouldn't mess around with it.

Also, if one of your fitness goals is to look more toned, then consider doing more heavy strength training rather than so much intense cardio, because you should be able to exercise less, and put less stress on your body, but get visible results more quickly. Not knocking cardio at all, because I think it's really important, but try to find a balance.

Normally you only lose your period if your BMI or body fat is very, very low. I had a BMI of 17-18 for most of my life and never lost it, personally. To me it sounds like there's a unrelated medical issue. I agree that you need to see your doctor because that's unusual. Feel better hon!

Firstly address you stress levels. Are they high or normal?

Secondly, I lost my period on 1900 calories and competed at 1800 calories, and I generally lose my period on anything under 2000 for long periods of time. Your body is under stress. You need to eat more then that, especially if you wish to train.

Hey Nikki
That's really really useful and interesting to know; many thanks for posting this. Over the course of four months I lost a stone (14bs) with very healthy/restrictive eating (around 1700cls/day) and working out six days a week with HIIT, weights, long runs, hikes, hot yoga, cycling and swimming. I have sustained this weight loss and my new athletic physique for the last year and am terrified I might go back to the "before" so might have pushed myself too hard as my period, which was once so regular, has become really irregular with long breaks in between.

I have never been overweight - I am 5ft7 and weigh 134lbs now and was 149 before - but through my eating/training plan have lost two dress sizes, lost 10cm (!!!!) around my hip and bum and have really nice definition everywhere. I certainly LOOK very healthy and far from underweight but my body fat is low and maybe enough is enough and I should go a bit easier on me again and stop counting calories.

For now, I will force myself to stop obsessively recording in MFP and ease off a bit. Thanks again for posting!