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I've always had clear skin, I would get an occasional pimple every once in awhile and it would go away on it's own. until this year I started noticing the right side of my face breaking out. I didn't think much of it because I thought it would just go away. of course, it didn't this all started around May. the right side of my face was just covered in pimples my skin was always so irritated after taking showers. I started getting face washes from the drug store in hopes that they would help. they didn't, and now the year is almost over and the acne has spread to the left side of my face. both of my cheeks covered in red spots, I don't know the name of these red spots. I've also been getting a couple white heads on my forehead.
I thought it was my makeup for awhile but I doubt it. I stopped wearing makeup for about four months now and my skin still looks just as bad. I'm turning 20 next month, and honestly this is making me so depressed. I don't know what to do, my parents are making light of the situation saying it's not that bad. which yeah it's not, it could be worse. but coming from a person who always had clear skin this is really new to me. I don't know where to start, I'm debating on the whole dermatologist thing. any advice? why am I all of a sudden breaking out now? 
I'm not allergic to anything, my diet hasn't changed. this may sound stupid but I was blaming it on a relationship I was in due to the excessive amounts of stress the relationship caused me. but that relationship ended in July, but obviously whatever is causing my sudden breakout isn't a stressful relationship. anyways, all the makeup I used I've always used but like I said it's been four months without using any sort of makeup. to add onto this I am on birth control I've been on it for about two years now. could it be possible that my birth control is causing my acne? I never had a problem with my birth control in the past though. please help 
Don't panic! There is a solution and you will find it. You just need to be patient and try what works for you.
You say you've had clear skin "your whole life" and then you go on to say you just turned 20... 
Lots of people have clear skin as teenagers and get acne in their 20s. I think we have all grown up thinking acne is only for teenagers but sadly that is not true.
I think visiting the dermatologist is the right thing to do in this situation. Tell them about the pill to, depending on which one you are taking that could be the cause of your acne. And careful with the acne products you use, as some can make it worse! Stay away from products that contain alcohol, you can have a look on this forum and you'll find plenty of products people recommend that aren't harsh for your skin. Moisturise really well as well and even though you said you aren't allergic to any food, it might be worth trying to stay away from dairy for a couple of weeks for example to see if anything changes? Dairy and gluten are often triggers.
Best of luck and keep us posted!
Your skin changes as you get older, so adjustments need to be made...also, sensitivities and allergies can come and go spontaneously, so you could have suddenly developed a sensitivity to something.
A few notes you may find helpful...skin on your cheeks is more delicate. It could be that your routine that has worked for you in the past, is now becoming just a little too aggressive and showing on your cheeks first.
(Always pay attention to the ingredients lists when using new products)
A common piece is advice is to change your pillowcase more frequently. Don't neglect to pay attention to what detergents you are using to wash them/clothes. One time many years ago, I discovered that I can't use Cheer...breaks me out.