Can I still claim JSA?

I am 25 weeks pregnant and still in the 6 month probationary period at my work.

I have not been meeting my targets, because of pregnancy related illness and stress.

Because I have not met my targets, I was given the option to leave on the grounds of ill health and with the option to re-apply after I have had the baby.

Or to enter final disciplinary improvement document, which would basically monitor my performance, which if unsatisfactory would end in my dismissal. However I have been on improvement files for the last month with no marked sign of improvement :( and if dismissed for poor performance I would not be allowed back.

So I was given these two options in my manager meeting today. I picked to resign.

It was only an hour later when all the paper work had come back and I was sent on my way that I realised the stupid error I had made, because I had chose to leave, rather than be dismissed at a later date - I surely wouldnt be able to claim jobseekers!!!

I only need to claim for 4 weeks, before my MA comes in, but I'm heartbroken, I feel like Ive been tricked into leaving :(

What can I do?