I Have Some Red Places on My Skin, and I Don't Know If They're Bites...?

I have some red places on my legs, and 1 place on my stomach. I don't think they're bedbug bites because I've checked my bed and didn't see any. I also don't think they're chiggers because I've had a chigger before and the bite didn't look anything like what I have now. Both my sister and I have them, but mine are worse it seems. My parents don't have them. It can't be anything we've ate because my family and I eat the same thing. The places only itch on my right leg. They look worse on my left leg, especially behind my knee. It kind of looks like blood blisters in some places. Behind my knee, it looks like a dark red bruise. I've rubbed Cortizone cream on them, and they haven't gone away. What are they?