I've noticed the past few times that I drank Dr. Pepper after 11 pm, I had pure acid come up. What causes this and how do I stop it?

You should read up on Dr. Pepper. You are only suppose to drink it at 10, 2 and 4 o'clock.

If you had 'pure acid' rising in your throat, you would have no throat left by now.
Acid reflux, heartburn, indigestion, GERD --- all extremely common conditions from either getting older, lying down, or eating certain foods. (If it turns into vomiting, then your doctor would test you for gallbladder problems, ulcers, Barrett's, and other less common conditions)

I had to give up all carbonated drinks over 15 years ago. I also have to severely limit what I eat or drink about 3 hours before sleep. Too many TUMS (antacid tablets) aren't good for you, so you may have to sit up frequently to burp, and learn how to make yourself burp.

Stop drinking this soda after a certain time.

You probably have what is called, Acid Reflux. You really should see your doctor about this as long term as over time it can erode your esophagus and even cause cancer in certain cases.

Its even possible you have a hiatal hernia, that allows this to happen ever easier. Another reason to be bring this to the attention of your doctor. There are medications that can reduce this as well as certain diets and eating schedules that will reduce it.

As was mentioned...reducing or stopping consumption of certain things as well as avoiding eating or drinking 4 hours before sleeping can reduce it as well.

But do see your doctor...there are various reasons for this and you want to know which ones apply in your situation.

Lets see: Dr. Pepper contains Phosphoric Acid? Yes. Dr. Pepper contains Caffeine? Yes. Dr. Pepper contains Carbonic acid? Yes. So, you have Hydrochloric acid from your stomach, Carbonic acid, Phosphoric acid, Caffeine, all of which can cause your symptoms. Then you lie down and GERD occurs. Skip the soda long before 11PM. Take an acid reducer.