You seem to be suffering from an acute attack of generalised urticaria
and could have been worsened by contact with grass if you have underlying grass pollen allergy
High-dose non-sedating antihistamines are suggested such as Allegra 180mg once daily, cetirizine
10mg at lunch time and evening for a week then lower to one tablet daily for a month.
If these recur after stopping antihistamines, you need to ensure secondary causes are excluded with a complete blood count
, liver function test, infectious screen etc.
1 in 5 people will have a generalised urticarial eruption in their lifetime and not necessarily due to allergy.