Hi...I agree with TomW...What you're experiencing could be caused by any number of factors.
In terms of the label, LD...I caution people to stay away from that since it is so pejorative...I know you need to use the label for certain accommodations in a school setting. But I find it much more empowering and effective to think about your issues in terms of how your brain is wired and what is happening in your environment at the moment.
I will tell you that, as a learning differences expert, I have not seen a case where someone has a strong academic history and then develops a writing problem in their early twenties like you're describing...You may not be able to take a battery of tests that traditionally would label you LD per se...The tests used may not be sensitive enough to pick up what is going on.
I would recommend three things:
1. Engage a highly-regarded tutor in your area that can help you develop strategies directly related to your transposing issue--your switching the order of your letters, numbers. As TomW said, it may be fatigue, stress, etc.
2. Get a thorough physical from your doctor. Just as a precaution and a baseline if this issue continues or worsens. Be sure to discuss any changes in your stress levels, physical health, major changes in your life...etc.
3. Have you had your vision checked? This may sound goofy but the point is that you really should rule out any obvious physical issues.
Keep me posted at doctormichaelhart@gmail.com. I also have other information on my blog at www.doctormichaelhart.com. Good luck.