As the other person who replied to you said, Sodium Lauryl Sulphate (SLS) is likely the cause, it was for me. Sensodyne do a Pronamel range which doesn't contain SLS and I've been using it for a couple of years with no issues as a result.
Apparently two brands make them, Cremodul and Trustnice; maybe a few others, but I'm not keen on using knock-offs because my teeth are quite fragile and apparently both of those brand knock-offs are much harder on the teeth. It also appears you can only really get them online from Amazon, which I'm definitely not keen on - out of the two, I'd rather throw extra money at Oral-B than a single £1 at Amazon.
Unfortunately I bought a brand new Oral-B iO shortly before I started boycotting American products and now I'm kinda stuck with it. I don't want to waste it, but at the same time I'll have to keep buying their expensive brush heads.
"It's always in the last place you look"
No shit?