Ammonia can rise for a lot of reasons. Given that you're 5 weeks in, your cycle is young yet. A little overfeeding could cause ammonia to rise beyond your young colony's ability to consume it. Again, though, there could be other reasons.
What's most puzzling is the drop in nitrates. These generally don't drop on their own and must be removed from the aquarium through water changes. Is it possible that your earlier nitrate readings were off? If you use the test strips, they can be inaccurate. If you use the liquid test, it's important to follow the directions closely and shake the bottle and the tube per the instructions.
Either way, cycles can stall and backtrack for a lot of reasons without us ever knowing precisely what they are. The only advice I can give is to keep testing, do your water changes as necessary, and be patient. Your tank will cycle.
Best!