When a doctor says wait 18 months before trying again - What does that mean?

Hi I’m looking for some advice please ladies! I had twins in my 20’s and waited 12 years to have baby number 3. He is an absolute angel. I got pregnant using this app. In saying that I was on here - calculated my data and never had another period. I got pregnant straight away! Same with the twins. I thought it would be harder because I was 38 with baby 3 (I must be fertile) but it wasn’t. As soon as I knew my high chance days I had sex and bam - pregnant! When I had him by c section the doctors said wait 18 months before trying again if that’s what I planned. At the time I’m like no way I am done, but when I was taking photos of my baby swing to sell online two days ago I got weird and started thinking ‘I can’t sell this what if I need it for more babies.’ It really threw me. I mustn’t be done if I’m having those feelings - right? And then out of nowhere I redownloaded this app and started logging all my period and details and stuff - all the while saying to myself ‘just incase you want more.....’ and then I started counting when baby would be here if I got pregnant now or in 6 months etc. so maybe I have made up my mind? Am I crazy? I’m 40, I’ve got 3 amazing kids and I’m happy..... do I want to do this again? Anyway feel free to comment on any part of this post but my main question to you lovely ladies is - baby 3 is 15 months on the 16th of this month. Does the doctors advice of ‘wait 18 months before trying again’ mean wait until baby 3 is 18 months old and then try? or is the 18 months including the pregnancy? Why do you think they say wait 18 months? Is it because I had a c section? If I decide to do this I need to get things rolling because like I said I’m 40 and I know the chances start going down around now to conceive and what else is there to consider having a baby after 40?

Thanks ladies