I finished my treatment in Apr 2018. My teeth were sensitive and have gotten much worse instead of better. What can be done?

I highly recommend you to go see your Invisalign provider to have your gum health and teeth checked. One should not be feeling all that sensitivity after Invisalign treatment. You may have teeth root surfaces exposed or suffering from periodontal issues which both can cause teeth sensitivity. Also both of these condition will not get better if you ignore them, they will only get worse. See a dentist ASAP.

I am sorry to hear that you have discomfort. Since your treatment was complated in April, my recommendation I to wear your retainers EVERY day for the first year. 
I am unsure if that is the cause..as a mature adult (68), there are certainly other possibilities that could be the cause. I would suggest an appointment with your primary dentist for a full evaluation and maybe some new radiographs.