My girlfriend says her body seems to go numb after I've been giving her oral sex for some time. Why does this happen and am I doing something wrong?

It could be that she’s too tense and breathing too shallowly. In which case she needs to chill, relax, breathe deeper into her abdomen and allow herself to let go and move and vocalise with ease. Or it could be that you’re just down there too long and you’re losing connection. You don’t have to do oral until she comes, in fact, it can be counter-productive. If you’re feeling the disconnect and numbness, change what you’re doing to something more active and where you can look into each other’s eyes to feel the connection again. You can always do more oral later in the session. Or turn yourself around and do 69, that will give her something to focus on!