Any advice for aggressive bees?

My brother is keeping some hives on my property the last couple of years and I've never had a problem being around them. Even mowing near by they always just kept to their business and I've kept to mine. But he just did a harvest and suddenly I have a bee chasing me around my yard as soon as I leave the front door, and have been stung several times by dive bombers. I don't even see them coming. He harvested the weekend before last and I figured they would chill out given some time. Is this true? Can I offer them a sacrifice or something?

This is in Illinois.