мъркам се

Etymology
Bulgarian Etymological Dictionary favours a grammatical origin from (implied also by Vasmer in relation to probable cognate 🇨🇬). Other hypotheses view it as a figurative sense of onomatopoeic.

Verb

 * 1)  to oestruate, to rut, to go into heat
 * 2)  to conceive, to get pregnant

Usage notes
1st person forms exist only in theory (unless used in an euphemistic sense by a person).