They are little bit painful. Your lips have over a million nerve endings. If you have a look at a cortical homunculus (one of those pictures of a body where the more sensitive parts are enlarged) you'll see that the lips are disproportionately sensitive.
This is why a lidocaine cream is used and allowed to act for around 30 minutes to numb the area as a local anaesthetic. The pain isn't crazy but included in the video below are some people having the treatment done (and reporting their feelings after!)
Fillers are hyaluronic acid that is injected below the skin. Your body produces it naturally too. This causes fluid to build up at the site of the injection and give the plump look. A typical treatment for the lips could mean a volume of 0.5ml to 2ml of the filler is injected. Clearly you'll fell this going in! But the pain is mild. It should only be light redness and swelling.
Although it's rare, there can be complications associated with fillers. If your pain is beyond some light local swelling and redness then contact your practitioner or a doctor. Over filling comes with the risk of restricting the local blood supply. This can cause some complications but the sooner you notice the better. You can check how quickly the blood returns by pressing the area and seeing how long it takes for the colour to return. If you're worried, talk to someone so they they can intervene. The sooner you notice the better.
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