Waverider's suggestion for a log is excellent. There are small pocket-sized books that are ideal. This should solve the problem.
Again, Waverider's suggestion to use your meter to ascertain your glucose status should positively determine if you require insulin. Keep your meter and plenty of test strips on hand.
I should add, do NOT inject again without first testing. You can cause a life threatening hypoglycemic episode with too much insulin.
Use your home meter to test your glucose levels. If you're having memory issues start a log - date/time/dose - so you have something to reference in the future.