Its scary at any age being pregnant, I had my first child at 18 and my last at 30 (and a few in between) and I was just as scared with them all.
The first thing you need to do is find out for sure. Go and get a test so you know what you are dealing with, either you are pregnant in which case you need to get prepared or you aren't in which case you need to look at what contraception you are using if you have reason to think it failed (assuming you aren't trying for a baby).
Secondly you need to stop justifying yourself, no-one needs to know how long your fiancee has known you, or even that you are engaged, if you are pregnant its your business and not something you need to apologise for.
People will talk, mostly to say how excited they are, or offer advice or help, you will get the occassional comment, but, again, thats something that happens with every pregnancy, people are too old or too young or won't be able to cope or are trying for a girl/boy or trying to patch up their relationship or a whole host of other things that stupid people will say regardless and if they can't be supportive then they aren't worth bothering about.
If you are pregnant then expect to spend the next 18 years 9 months fretting about every tiny thing, its all part of being a parent but please do find out for sure, sooner rather than later.