Overheating problem only with me, it never happened with my boyfriend.

Hi, we purchased this vehicle last month. It ran fine for like the first 3 weeks (I do grocery delivery for a living so i'm running it a lot). It had started to overheat (only with me, it never happened with my boyfriend.) We checked the radiator and it was really low so we filled it, reservoir is also filled. It ran good for a couple weeks then it did it again with me, so we checked the radiator and was again low (but reservoir was full). So we went to get fluid when we went to fill radiator was full again. But low the next day. No visible leaks on the ground or inside, and we had head gaskets checked but it all checked out. So we filled again and checked every time we were about to drive and it stayed full (last filled may 3) until I worked today. It was low again. Didn't really overheat went up slightly while idling in a drive through but not much and immediately went down. But im paranoid so I checked once car cooled down and it was really low. It's not happening consistently so it doesn't seem like the overflow is pulling consistently. Every mechanic I have talked to just sends me somewhere else. I guess they aren't understanding me?