Bitcoin is completely transparent, with all transaction visible on the public ledger. Your statement “I imported my private key, and the wallet showed I had exactly…” suggests that you're making some incorrect assumptions about how Bitcoin works.

You don't need a private key to find out where payments have gone from your address — just look it up on a blockchain explorer web site to find what was paid to where and when. Incoming payments are also visible and traceable. Also, if you've made incoming payments yourself, you could use the same blockchain explorer web site to look up (i) the sending addresses and (ii) the addresses to which bitcoins were sent from the sending addresses.

Once you've tracked down all relevant incoming and outgoing addresses, you need to see which of them are in one of your wallets. If someone else has made an outgoing transaction from any of your addresses, then that person has gained illicit access to one or more of your private keys.

You will not get a definite answer until you've checked all of the sending addresses and destination addresses.

EDIT: You should avoid exposing any of your private keys to the internet until you're clear about how Bitcoin works. For large sums, it's advisable to keep the private keys permanently off internet-connected computers, and to learn about cold storage techniques that allow offline signing.