Why has my stutter come back over the last two years? And why do people treat me like I'm unintelligent because of it?

Why has my stutter come back over the last two years? And why do people treat me like I'm unintelligent because of it? From the first time I could speak, I developed a stutter. I was in speech therapy for a very long time as a child. It seemed to help a lot because I was able to get by without stuttering every second word. It never fully "cured" my stuttering as my mom would put it. But it definitely helped a lot. I recently started a new part-time job (i'm 20,) so it's not so strange that I've been a bit nervous, which is perhaps making my stutter worse. Although, my stutter has just been overall becoming more prevalent over that past few years. I think that some of my coworkers think I'm unintelligent because of it, they sometimes speak to me very slowly. I overheard my manager saying to the district manager, "She's doing really well, we're working on her customer assistance, but she seems to be stuttering a lot, which is fine I guess. We will work on it" Today one of my coworkers even made fun of me when I stammered, then they finished what I was trying to say. Then began to explain something slowly to me. A lot of the time people just don't let me finish and speak over me. Do other people share similar experiences? It's a super awful feeling to be heavily underestimated, due to something that doesn't reflect my intelligence. The more I seem to try and work on my stutter, and getting word jammed the worse it seems to get.