Hello,
If you had abdominal pains and nausea the very next day after having unprotected intercourse, then it is unlikely that it is due to pregnancy. It can be due to gastritis or GERD. Try raising the head end of your bed by at least two blocks and take your meals 2 hrs prior to bedtime. Also avoid taking two or three heavy meals. Instead, take regular frequent small meals. Avoid taking tea or coffe prior to bedtime and concentrate more on a healthy diet.
If the symptoms persist then an examination by a physician is desirable.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
One- symptoms usually take a couple weeks to come up. So there's a good chance that the pains had nothing to do with it unless you had unprotected sex earlier.
Two- "early pregnancy symptoms" like nausea, fatigue, headaches, back pain, cramping, appetite change, bloating, and even period irregularities can all be caused by other things. For example those same symptoms I have felt at different times due stress and/or my thyroid problems. So even if you have these symptoms the only way to know for certain is a positive test.
Home pregnancy tests should be taken 2-3 weeks after possible conception with the first morning urine for most accurate readings. Because many things can cause these symptoms if any of these symptoms get bad you should see a dr to rule out anything serious. Also it's never a bad idea to eat healthy - for now just make extra sure you avoid all drugs and alcohol. If you're on a prescription drug that could cause problems talk to a pharmacist or dr. Many pharmacies have 24 hour lines that you can call, and no question is too stupid for them to answer.
If you're not pregnant and not trying to conceive I suggest you go to further lengths to avoid an unwanted pregnancy. Get on birth control and, because no birth control is 100% effective and only a condom will prevent a STD, use condoms as well. Just remember condoms break too so really multiple layers of protection are the way to go.