If you are only milking her out once a day and she has until very recently fed a kid I would definitely milk her twice a day, especially if you want to continue having milk from her (and she is not in kid).
I am not so sure about mastitis, she could just be very full on her side due to lack of kid suckling - 2.5 pints (about 1.5 litres???) per day mainly from one side is quite ok for this time of year.
If it is mastitis you will find that a) the milk yield will continue to reduce and b) when you filter it it will go quite slowly through the filter (something to do with the bacterial load). You can also do the "washing up liquid" test - just squirt some into a cup containing a bit of milk, if it all goes stringy and goey as you mix it you will have mastitis. if it just foams as normal - no mastitis. If you try and get it tested by the vets/laboratory - it costs about 100 pounds to culture etc etc.... First antbiotic I would try would be Pen & Strep. If she has small teats do not use the antibiotic stuff that needs to be squirted into the teats - better with an injection into the muscle.