1st trimester to 2nd

It's so confusing !!! I thought 1-3 months is 1st 4-6 is second and 7-9 is third .. doctor said I wouldn't be in my 2nd trimester until I'm 16weeks but people on here is 13weeks and happy there in 2nd trimester already I'm so confuse !!! But not rushing trimester either lol I want this pregnancy to go slow cause my first went by to quick I didn't really enjoy it has much 😩😩