Using Weird Memory from Tomcat

Vague question. Therefore, please do not hesitate to ask for any specific details.

We have a tomcat server running two web services. One cat is built using spring. We use mysql for 90% of tasks and mongo for jsons caching (10%). Another web service is written using the grail. Both services are medium codebases (about 35k lines of code)

Computation only occurs when there is an HTTP request (no batch processing). Approximately 2000 queries to the database on demand (I know this is a lot. We are working on it). The request rate is about 30 req / min. For one specific request, there is image processing which is quite expensive. No JNI anywhere

We found strange behavior. Last night I can confirm that there have been no requests to the server for about 12 hours. But when I look at memory consumption it is very confusing:

Without any requests, the memory continues to jump from 500MB to 1.2GB (the 700MB jump is a concern). As mentioned, there is no computation on the server side. I'm not sure if there is a memory leak:

• Memory usage goes down. (Everything would be easier if the memory did not fall).
• This behavior is reproduced with SoftReference-based caches or so. With full gc. But I don't use them anywhere (not sure if something else uses)

What else could be the reason. is this a cause for concern?

PS: We've had problems with memory crashes (not bugs, but JVM crashes) quite often quite recently.