Yes, but way to early to tell.

Yes, having sex is what makes you pregnant. And since no form of birth control is 100 percent there is always a chance.

Yes you could but if you are worrying now before a period has even been missed then why did you not use a form of birth control, it is however far too early too tell so the only way to find out would be to wait until your period. If you are asking this question because you are trying to conceive then you need to stop worrying and running it over in your mind ntil your period is due as all these factors can delay your period