The answer to all of you: YES you could be pregnant. Whenever you are having sex, you could always get pregnant. However.. For the sakes of the questions in this string....
If you are pregnant, it happens before the last full cycle, not this current one, so if you had sex 4-5 days before, you would not be just 4-5 days pregnant, you would be around 5 weeks pregnant. The symptoms start around the 3 week gestation mark, when the pregnancy hormone HCG is starting to rapidly multiply.
Some points to ponder: Dizzy feeling and nausea can generally go in hand they can be characteristics of a slight flu. Also, rapid changes in blood pressure can cause the dizziness, and nausea, dizziness, fatigue can all be symptoms of diabetes.
Overall, could you be pregnant, yes. Any time you have sex you can get pregnant. I am currently preggo with my 3rd baby, and found out earlier this time than last, I knew when I was 3 weeks along, and doctors even told me no, and when I went back 4 weeks later, ultrasound said 7 weeks- I said, told you so

The body is an amazing thing, and the hormone elevations can cause many problems, but it would not be right away. I suggest, if you had a prior full length, average period over 30 days ago, then take a test. If not, wait a week and take a test. The dollar tests work amazing for accuracy. All you need is something to detect HCG, which a 1$ test will do just fine for that. If its negative, wait 2 weeks and retest. Also, test in the morning when HCG Levels are highest. If it is positive, get into a local clinic right away to begin the prenatal care, and get on a good vitamin. It doesnt have to be a prenatal, take 2 flinstones (kids vitamins) if you wish. Thats all I have ever taken

and just make sure you get enough folic acid... And, I am wanderin around here, so message me or talk if you need me