Bree is a 16 years old british girl that posts videos on youtube about anything having to do with periods and reusable menstrual products. Her youtube channel is seriously awesome :) she even has a video explaining menstrual cups and reusable pads to parents that I suggest you show your mom. Just tell her you're not comfortable with using tampons, hopefully she'll understand. Here's a link to the video, good luck :) https://www.youtube.com/watch?v=sbnpvmbFQoM

 Show your mom an article you found online about menstrual cups, or a video on youtube.  And as for wearing a tampon in the shower, you don't need to.  I don't (I do perfer tampons over pads when I'm on my period, except for at night or in the shower or bathtub).  The blood goes down the drain.  If you're going swimming while you're on your period, then yes do wear tampons or menstrual cup.  Unless you're on the last day or so on your period, because then you hardly have a flow.  as for the TSS, if you take the precautions suggested on the pamphlet in the box of tampons, you most likely won't get TSS (although it is still possible, and I think I have even heard of someone getting it from changing her tampons too often).  

 there are some videos on youtube about menstrual cups.  Perhaps you could show your mom some of those videos, and then the two of you could talk about this afterward.  If you don't feel comfortable going right to your mom and saying, "Mom, I want you to see this video," perhaps you can email your mom the links to some of the videos, if she has email.  That's what I do.  I haven't emailed her any of the videos about menstrual cups, but I have emailed her other videos and articles.  I have Asperger Syndrome (High-Functioning Autism), and I have emailed my mom the links to some videos and articles that have to do with Autism Spectrum Disorders (such as a potential treatment that claims to cure Autism).