Casual sex without using a condom can definitely be a source of some sexually transmitted infections including HIV/AIDS. If you are worried, it is quite natural. We suggest that you wait for a week or so more for any symptom to appear (e.g., burning/pain on urination; discharge of pus or blood in urine; frequency of urination; appearance of some ulcer or sore on the penis etc.). After one week, whether symptoms appear or not, you need to consult a qualified venerologist personally and seek opinion. You would be required to undergo a few tests to rule out STDs. As regards ELISA test for HIV, that can only be undertaken 3 months after exposure. We would also advise you not to indulge in unsafe sexual practices.