Hello
Welcome to health care magic.
I understand your concern.
It is certainly
allergic reaction to the environmental allergens as you said you work in yard.It can be allergic reactions to the poison ivy,chemical insecticides if you use or the
insect bites.
It is possible for rash to develop after 24 hours of exposure.
If I were your treating doctor,I recommend,
-Antihistaminics like oral
cetrizine or
benadryl for 3 days.
-Apply ointment containing1%
hydrocortisone for 3 days.Also applying local ice compresses will help in reducing inflammation.
-Always cloth your hands and legs completely while working outside.Can use insect repellant on skin for protection against mosquitoes.
carry out regular deweeding and cleanliness of your garden.
If it still persists beyond 3 days,I advice visit to physician/allergy therapist.
Hope this information helps.
Any more doubts will be appreciated.