I think it's either Japanese Clover (pink or purple flowers) or Black Medic ( yellow flowers).
A few things...
Both thrive in low nitrogen lawns with compacted soil.
Both are annuals that reproduce by seed only. Preemergents, like corn gluten, work on lawns to prevent the seeds from germinating. It has to be timed right, and usually takes 2 applications in the spring. Corn gluten also feeds your lawn, so it's dual use!
They are both pulled out by hand very easily.