Its still possible that you have a virus that’s using you to defeat captchas so it can use the answer for spam purposes. Check, and re-check, and Google and re-check, is what I’d do, simply because I hate being used. YMMV.
I have a cheap TMobile plan, I only have like 5 Gigabytes a month, and then another 5 from using my phone as a hotspot. I may have more, like 7.5 each, or something. Doesn’t matter – towards the end of the month, they start clamping down my rate. That can make bogosity like this turn up for me as well. I get captchas from Google, or I get notifications that IP isn’t secure, or the the SDMB isn’t configured for secure browsing. But the problem goes away after I switch to the free internet from the library. So I think an internet slowdown may make your browser, or perhaps your ISP act weird like this. So see if it goes away when you have really good internet from another source.