Is there any chance of getting pregnant a week before expected period if so how much of a chance?

Relatively Low: Usually by that time the egg has ovulated and is no longer around to become fertilized by the sperm. However, there are reports and experiments showing occasionally eggs may stay around longer. And it is possible to have ovulated later than expected. So the chance is overall going to be low to become pregnant, but there is no way to quantify it. If i had to estimate less than 5%.

Yes: That's not the usual timing, but it's stil possible depending on your actual cycle. If 23 years is your correct age and no fertility problems for you or male partner, chances might be around 20% per cycle.