Since your periods are irregular it is difficult to say when to undergo the pregnancy
If at all you want to get it done it can be 8-10 days after the unprotected intercourse
earliest but if negative you cannot be sure since the hormone levels might not have rose upto the mark to give a positive test.
You can go for a sonography
also but the exact time when to go is difficult to suggest as your periods are irregular.
Dr. Rishma Lakhani