Rail fares rise in January every year. The average rise this year was 3.6%
http://www.nationalrail.co.uk/times_fares/ticket_types/83871.aspx
If you're seeing a different price between last month and this month then it must be because (as Hopkirk suggests) you're not comparing like with like.
If you travel into London with an arrival time before 1000 you'll be charged a Peak fare. Similarly, if you're returning from London between 1629 and 1834 you'll usually have to buy a Peak ticket (although there are certain exceptions). Outside of those times you can get Off-Peak tickets. Further, on certain routes you can get Super Off-Peak tickets if you arrive in London after 1200 (although you're then restricted to returning from London outside of a period extending from 1559 to 1901).
[Note: The times I've quoted apply on my local line into London Liverpool Street. They vary across different train termini and rail operators].
Remember as well that the price of Advance tickets (which are only sold as singles, not returns) can vary greatly, depending upon the popularity of the service you choose to use and how far ahead you book.