Yesterday I sent an email explaining some seo techniques to a client and today I got 2 emails from other gmail addresses that are clearly phishing scams that want me to purchase seo scripts. I do know Google displays ads that are relevant to emails in your inbox which I can't seem to be able to turn off. Is it possible for other people to see what criteria Google uses to display ads to its users? I don't think my account was hacked because the only IP address that has accessed my account within the past 30 days is mine.
Reading from your description, I believe that it is due to the fact that you have advertised your email address to other people, which resulted in you receiving those SEO-based email spam.
This might not necessarily be the case though, so I can believe that it is something up with your client or maybe its just so coincidental.
Note that Google does show ads on their Gmail based on the contents of your email, but does not send you spam mail to advertise products (why would they when they are implementing spam filters themselves). Also, receiving such spam isn't related to other people signing in to your account, it is a normal email sent from someone else's email account.