I need advice about engagement

My girlfriend and I have been together for 15 months and she recently bought a ring and is going to propose sometime soon. We have lived together for awhile now and have been through some major events in the last year such as me getting off of a really bad anxiety medication, having to move, family member got diagnosed with leukemia, and a lot of other things. I don’t feel like it is to soon but I had someone tell me that it was and now I’m not sure. I know she is the one I want to spend the rest of my life with and she did ask my dad and he said he had nothing negative at all to say. My dad isn’t someone that takes this lightly either and he did tell my sisters husband no the first time. When my girlfriend told her dad though is only response was “prenup”. I had already asked her dad to marry her and he was really happy about it and has supported our relationship, but now I don’t know why he made that comment. He has been married 3 times, twice to the same girl, and one did end very badly so maybe this is why he said it? I felt really offended by it though. I have never taken anything or used my girlfriend at all and it’s not like she has much that would go in a prenup anyway so I was confused by it. Do you think it’s too soon to get engaged? And what do you think he meant by that comment? Would you be offended to?

*my girlfriend has been married before. It was a military marriage that just didn’t work in civilian life.