I call it "mommyitis" It is very common with new moms. It is caused by the repetitive motions from diapering, buttoning, feeding, etc. Best thing you can do it to keep icing it and invest in a brace and not the wrist kind. You mentioned it is at the base of your thumb, get the one that is made to stabilize your thumb. Don't wear it during the day, wear it at night. When sleeping, we tend to curl up our wrists/fingers which puts continued strain on all the tendons that go through them so it never gets a chance to rest. It will take several weeks for it to start to feel better, but if with icing and wearing the braces at night does not start to feel somewhat better in about 1 month's time, go see an orthopedic doctor. They can give you cortisone shots to take away the inflammation, but the only problem is if you continue to do the same things, it WILL come back. Now that baby is at least 10 months old, she should be holding the bottle herself, so it relieves that stress. Keep her in skirts all the time instead of onesies or pants so that you are not having to unbutton and button all the time during diaper changes. Little changes like that should help.