Actually this is QUITE common - women who exercise a lot - such as women like you training for a marathon, Gymnasts, etc - they actually stop having a period because of lower than normal BMI which also decreases their estrogen levels a bit - NOW there is NOTHING wrong with that - and actually it helps in the long run for various reasons. But once you get back to a normal life without all that training - your BMI should increase a little bit, your estrogen levels should return to normal and you should have normal periods and you should have no problem getting pregnant.
The ONLY things I would check NOW are a thyroid and a prolactin blood test - just in case those are the reasons why you are missing your period - as these can be easily treated with medication.
I wish you well
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So its fair to say I have nothing to worry about
That is correct -
great, thanks so much
IN my answer I have given you everything you need to know