It's only correct if: 1) Your pants have been washed. 2) You thought person A washed them. 3) You learn person A did not wash them. 4) You want to know who actually washed them.
NOW it is correct to say "Who did wash my pants?" (Or "But who did wash..." or "So who did wash...")
However, I think this is actually a possible translation of the Spanish sentence, so try reporting it.
Vadim, it's correct but unusual. Normally, when you're asking a question with "who" or "what" to find out the subject of a sentence (i.e. trying to find out who or what is actively doing something), you won't use the "do" auxiliary:
- Who wrote this? - asking about the subject, the writing person
- What did he write? - asking about the object, the written text
- Who called you? - asking about the subject, the person calling
- Whom did you call? - asking about the object, the person being called
- What do foxes eat? - asking about the object, the animals that are hunted
- What hunts foxes? - asking about the subject, the animals that are hunting