What that likely means is that just over 30 days ago you got two upvotes. That no longer goes into the 30 day but still counts towards your total. I would however recommend strongly on focusing more on the message that your karma sends overall in terms of what comments have been received well and which have not rather than look at minor fluctuations.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.