You will have had one strong flashback which made you conscious of it happening now other small ones (that happen all the time) seem more regular and vivid.
Essentially your brain doesn’t really forget anything, it just gets buried away somewhere at the back – provide the correct trigger and it will reappear in your consciousness.
I always get a really strong visual flashback whenever I smell a real Christmas Tree – I can almost see myself walking into the front room of the house I grew up in on Christmas morning. I also get them when playing music – occasionally I hear a track I haven’t heard in a long time and I get transported back 30+ years to sitting in my bedroom listening to Ratt or some other obscure rock band of the time.