What Causes A Hard Lump Of Blood During Menstruation?

im am currently on my period. my period is never of a consistent length of time always being a different cycle length. this month it is 31 days where as last month it was 32, im on my third day and although painful not as bad as usual, yet i have just had a gooey but hard lump of blood come out of me, could you tell me what this is?