There are many reasons why this could be happening. It would be helpful if you could fill us in with more details. From the sound of it, it could be a stage, teething or even night terrors as you mentioned hard to calm him down. However it is hard to define what the issue could be. Sometimes, it happens for no reason too, just oart and parcel of the parenting. I went through a similar stage with my firstborn. I got scared h8nestly, thinking if it was something paranormal. Oh well, please dont allow me to scare you haha.
I have attached a link for you, similar scenario based. Might be useful to you.
http://www.parents.com/baby/sleep/issues/teach-baby-to-sleep-through-the-night/
Dont be too hard on yourself. All the best.
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