My sister is experiencing unusual blood clotting during period?

she doesn't normally have problems like that. I'm worried because I walked into the bathroom because she needed new underwear and the clot was the size of a golfball, may e bigger. she's 23 and she had gastric byp in December but is recovering well and idk if that would make a difference on that. she's got a doc appointment for Tuesday....next week...I'm just worried. is this normal