Sorry to hear about that. I’m at work now so can give more detailed advice 😉
The treatment provokes a slight inflammatory response. Anything inflamed tends to swell up but the area around the end of a root is all enclosed in bone so there is nowhere for any swelling to expand into – hence the pain. Normally this slight inflammation is controlled by normal painkillers.
Sometimes, usually where there is a chronic infection present (where the bugs and your body’s defence is in equilibrium) it can be stirred up to become acute, sometimes called a phoenix abscess. Likewise an acute infection causes swelling and hence pain but it tends to be a bit more severe and often needs antibiotics. It doesn’t mean the treatment will not be succesful. Generally, 24-48 hours of antibiotics will make a significant impact on the pain and any swelling.
HTH