Help with problem neighbor
We live in the 3rd floor, the top apartment of my building, we are two professionals, my girlfriend have a Masters and I have a Doctorate. Me and my girlfriend have been living here for almost a year now. Lately the downstairs neighbor have been calling security, because of "loud sex noise" and "showers in the middle of the night" this comes to a shock to me, and I'm thinking that it might be someone else. But the incident last night makes me think that she is complaining specifically about us. In order for us to engage in this activity it has to be before 9:30pm because once my girlfriend is ready to go to bed, is not going to happen, so we usually have sex before that, and we are not screamers. It is normal sex. The thing is that the security come knocks on my door two to three hrs after this, and another time at 1:00am when we are already sleeping. I understand that with apartment living there are some noise that is going to go through walls and floors. She might be hearing some bed noise or something, I really don't think is any other than normal walking around noise, and ever since this started to happened we have been trying to keep the noise level to a minimum just in case, and to be a good neighbor. But what I am supposed to do? Stop having sex or have sex only when is convenient to my neighbor?
My question is: What can we do?