I found another woman's panties in his wardrobe

I Found Another Woman’s Panties In His wardrobe , We Have Only Been Dating For 8 Months.

I have a boyfriend and we have been dating for 8 months. One Saturday morning i went down to his place from work as i did the night shift. Later in the morning i went into his wardrobe to get a t-shirt and then,i found a pair of used women’s panties that did not belong to me. I called him out and asked him whose panties it was and what it was doing in his wardrobe.
He said they were an old girlfriends from about a year ago. I was shocked ,i had never seen it there or i want to believe that i probably don’t notice it was there. Then he asked if I kept any mementos(an object kept as a reminder of a person) from my past relationship(s) and if he should throw them away. I screamed at him to get rid of it.
I couldn’t stop myself from thinking about what happened. I am really concerned about why he kept them at all. I really do not think he is cheating. Just wondering what this means? Do guys really keep stuff like panties? or other mementos from your past relationships or am i just overreacting.