Brownish discharge usually indicates old blood or a small amount of blood which has got oxidized. Meaning thereby that some small amount of bleeding did occur. The cause of this at 38 weeks of pregnancy
could be the beginning of your labour pains, which you will know by and by as you may go into labour. "Show' may appear in different forms in each pregnancy although classically it is a blood stained, often mucoid discharge.
There is a condition called Accidental Haemorrhage wherein the placenta may separate a bit leading to bleeding. I'm co-relating this with your history have having done physical work. In some cases all of the separation of the placenta may not be 'revealed' by bleeding which shows. However, since you don't have any tightness or pain abdomen and the baby is moving well, there may not be anything to worry about although i would suggest that you call your doctor's office and inform them. Your doctor may decide to have an ultrasound
Other than that there could be some infection unrelated to your physical work. If any further discharge comes on you may need treatment for that.
Hope this answers your query. Take care.