Is this withdrawal or something else? cross tapered Mirtazapine to Sertraline

Hello all,

I am wondering if someone can identify with my situation.
I was initially on Mirtazapine 15 mg for sleep anxiety for about 6 weeks, Turned out it was not helping so GP put me on Sertraline 50 mg. I felt better instantly and my sleep also improved. Then he stopped my Mirtazapine cold turkey and no issues what so ever. I did some heavy exercise about 7 days after i stopped mirtazapine. I suffered horrible hypnic jerks all night which reduced and finally stopped in 4-5 days and my sleep returned to normal. Then the other day which is probably 3-4 weeks since i stopped mirtazapine, i had massive anxiety and those jerks returned. They were much reduced last night,
What exactly is this? Is it withdrawal from Mirtazapine or simply Sertraline mistaking my exercise for anxiety. My gp had no clue about withdrawal symptoms and he said it might be and wanted to raise my dose.
I am not 100% sure that a dose increase would help but i have been having fits of crying and general low mood. Is this experienced by anyone?

Please help !!!