Ovarian stimulation is done with hormone medications, which can cause some side effects. They are not painful or specially severe, though. By no means they are symptoms that could hinder the donor from continuing with her everyday routine.
Commonly, headache, bloating, tiredness, etc. are the most common symptoms during the days leading up to egg retrieval.
In case the donor develops OHSS (Ovarian Hyperstimulation Syndrome), the symptoms derived from it can be moderate to severe. The good news is that only a small percentage of women develop OHSS, and it can be often prevented through ultrasound monitoring. You can get more information about it here: Ovarian Hyperstimulation Syndrome.