There are all sorts of things it could be and you should suspect before Thunderbird. So some first questions:
Have you checked your ISP and email provider's service status pages? If not do so. Does it happen everytime you send an email or vary in any way? E.g. time of day / week Have you stopped any torrents or other P2P you're seeding? If not do so. Did you make any changes to your home network / WiFi in the run up to it starting to happen? >From your email address it looks like you might be using an email provider rather than your ISP's facilities. If so have you tested via another Internet connection? E.g. plug an Android phone in via USB and turn tethering on, take a laptop to a public WiFi hotspot. If not do so.
