What are my chances of contracting HIV after unprotected period sex?
If your partner does not have HIV: zero. You can never ever ever ever contract HIV from someone who does not have HIV. If your partner does have HIV but is on antiretroviral treatment that reduces their viral load to undetectable levels: hypothetically not zero, but so close to zero as to make no nevermind (to my knowledge there has never been a documented case of transmission from such a person). If your partner has untreated HIV with detectable viral loading: Low, but definitely not zero. Testing would be prudent.