Changed IP pool

Hello there! 

I recently had a dedicated IP assigned to my account, so for some time I was sending emails through one shared IP with a good reputation and a dedicated one, which had medium reputation at that time. 
I am also monitoring my deliverability through Inbox Monster and Postmaster tool to detect any changes that might be harmful for the reputation of my domain. 

Just within a week after sending emails from those 2 IPs, I noticed that the pool of my IPs was changed. Instead of seeing a dedicated IP and a good shared IP, I started sending email from 3 shared IPs with low reputation. 

Did anyone experience such unexpected change of IPs for their account? What might be the reason for it? 

Thank you!