Should I be worried?

Hi guys. So just a little background information before I get into the point of the post.

I'm 21 years old and overall I'm pretty healthy. No known medical problems not even asthma I've never even broken a bone. I'm 5'7 and 150 pounds which is average. I don't drink and I smoke occasionally but not often. I work and live with my boyfriend, I have a wonderful family (with no known fertility issues on either side of my family) and I live a nice stress free life at the moment.

Now I've been with my boyfriend since 2015 and we've been having unprotected sex for almost 18 months now. On average I'd say we have sex every other day and during busy weeks maybe 3-4 times. Yes we've had sex during ovulation and even though we aren't necessarily trying to get pregnant we are NOT not trying to get pregnant & I am surprised I'm not. My period in 2015 and the beginning of 2016 was super irregular ( in 2015 out of 12 months I got my period 7 times and also in 2016 in 12 months I got my period 7 of those months ) fast forward to 2017 we are currently in July and in 7 months I've only missed a period in march but for the most part it's been on track.

Why am I not pregnant ? Should I be worried ?

& btw every time I bring this up to my doctor he tells me that I'm young and shouldn't worry about having kids right now but it's not that I'm worried about having kids I'm worried about my fertility health because the sooner the better you know. Any suggestions on what I should do ?