Since I returned I feel like I’m being punished for having had a baby

I work in the London office of an American company and since I returned I feel like I’m being punished for having had a baby. I had been due to leave on the 13th of December as my baby was due on the 18th and I thought I’d work for as long as possible, especially as the London office had been going through changes and I felt I was one of the longest serving employees who knew how things ran. My daughter came a couple of days earlier and as expected my maternity leave was changed from that day. No problem there, the only thing was that I was never told. My work laptop was requested and communication with the company was ceased whilst on leave.

When i was due to return, luckily I confirmed with what i thought was plenty of time, turned out I only just gave them the correct notice. I was sent a letter being told that the girl that they hired, which i trained to be my cover, had been given the job i had been doing. Although i was never formally office manager i had been doing the job for a year (without any complaints from anyone). So I was effectively demoted to the most junior position and now duties which i did when i first started have also been taken from me. Now the CEO of the company keeps calling, hammering down that i now report to the cover girl with thinly veiled intimidating tactics. Every time we speak he pretty much always mentions that I am free to leave. All in all, this has been so unpleasant and stressful, when i have always been hard working and loyal to the company.