There are probably many reasons why this is happening. Like you mentioned, she could be scared/nervous, she could've been handled incorrectly, or be bored. I can probably explain some of these, but i could be wrong. Hammies are hard to understand why they do things, but sometimes you can tell. If she's scared, she probably needs space sometimes. If you left her alone for a while, she may have gotten used to being alone and doesn't want to be handled all the time. She may be squeaking because she is nervous that you are going to take her out again.
That could be a reason but maybe not. She could be squeaking because she is bored. Even though she has a 5 ft. cage and many toys, she probably wants to run around. Hamsters love space to run around in, a wheel isn't always enough. Though i mentioned she could be nervous that you are going to take her out, when you spend one on one time with her she's probably like "if i'm out of my cage, why can't i go for a run?". When being out of the cage, hamsters just want freedom to explore. That's why they sometimes escape from their cage. Once in a while, put her in a playpen and let her run around or put her in a ball. Don't bother her while she's doing this or she might get annoyed. Again, she could be used to being alone.
Since she was squeaking right when you brought her home, there's a possibility she could be sick. Pay attention to her behavior. Is she eating enough, drinking enough, sleeping enough? You can probably take her to a vet. If you have tried everything i listed above and she still squeaks, a vet can help.
If nothing works, she could just like to squeak! You never know since hammies are unpredictable. Hope I helped!