Firstly. Irregular periods are common even 2-3 years after you first start. Just because you have had relatively regular menses up until now does not necessarily mean that there’s something horribly wrong. [sure, there are exceptions to that rule, but generally it’s totally probable that you’re okay]
Secondly. There are tonnes of reasons for irregular periods. Here are just a few. Ask yourself these questions. - Do you have exams coming up? Do you feel stressed through anything? - Have you had a recent diet change? - Have you started taking oral/injected/implanted contraception? - Are you on any hormone related medication? - Have you switched contraception brand recently? - Have you had unprotected sex?
These situations could lead to loss of/ irregular periods. Your body is quite sensitive, since hormone/diet changes can affect your cycle. If you have had unprotected sex, you don’t necessarily have to be pregnant, you could have contracted an STI, chlamydia for example.
Okay. Now for some solutions.
For peace of mind, you can take a pregnancy test. These work generally from the day your period is due. clearblue -  - is a reputable test company [now with free demo!]
Alternatively, if you can get to a clinic. brook clinics -  - are fantastic and geared towards helping people in your situation, quickly and confidentially.
Alternatively, there ought to be an NHS clinic or even a youth clinic in your area that can help you out for free. And what’s more; they won’t alienate or make you feel silly.
The same clinics can also help you if you fear you may have contracted an STI.
And lastly: If you think you have an STI or think you are pregnant, avoiding the issue won’t make it go away. Nothing like this disappears in time, in fact, it generally gets more difficult the longer you leave it. If you’ve missed a period, and have had sex recently, nothing could put your mind and body at rest better than a visit to your local clinic. They’re there to help you as much as humanly possible. No amount of internet forum reassurance can do that for you.
Also; nothing beats adequate contraception for peace of mind during and after sex. Don’t be forced in to anything you’re not comfortable with or will regret in the morning.