I would suggest a sturdier band - perhaps one like that used for carpal tunnel. They can be purchased at any chain pharmacy or surgical supply store for about $15-20.00.
NSAIDS are your best bet for the inflammation, but at first they may need to be taken regularly around the clock and in higher than usual doses.
Anything can cause tendonitis, but often it is a repetitive motion that does it - something like using a stapler more often than usual.
Unfortunately, the tendon can remain inflamed for several weeks or even several months!
Here is a site devoted to the topic - don't intend for you to purchase the products advertised!
Rest is the best treatment and the additional support of the higher quality band may help. Here is a picture from the Walgreen's site.