first off, if you are worried, stressed, thinking, etc. that you could be or are pregnant, it can cause "symptoms" like pregnancy symptoms.
Our bodies react to stress in weird ways, and sometimes when someone thinks that they could be pregnant their body starts telling the brain they are pregnant and then symptoms of pregnancy show, when really there is no pregnancy occuring at all.

I found out that i was 100% pregnant by, taking a digital read pregnancy test, and getting a blood test done to confirm that i was indeed pregnant.
Before i tested, i had a feeling i was because i felt really off. I had headaches alot, i was boiling in winter and im always cold, i didnt feel good but not a flu kind of feeling, but i tested to set my mind at ease instead of stressing and wondering why i felt this way.

But, do not read into how i felt, and analyse your symptoms based on someone elses, because everyone feels different things while pregnant, and they vary from person to person.
Also it could possibly make your mind wander more and make you think that you are pregnant.

Your best bet is to take a pregnancy test to be sure or see your doctor and ask for a blood test to be done to determine whether or not your pregnant.

How long has it been since you had sex and when are you due to expect your period?
You can use a pregnancy test 5 days before your expected period and get a positive read from it.

I hope this helped, if you have any other questions PM me anytime.