It's a streatch but it's possible there's an underground stream. As to why it's all-of-a-sudden happing now IS a 'mystery'.
Maybe you need to have the home inspected. Since you recently had it inspected (did you?), call the inspector up to see if he'll revisit your home. (If it's an ASHI-Approved Inspector, then he should come out at no charge.) DO check your inspection paperwork where it 'talks' about the basement. You may find some info in there.
If the gutters and downspouts and landscape are OK (taking your word for it), it's possible that if there's a foundation drain or french drain or something already existing outside, it may be clogged, thus backing water into the basement. If you can ctc the previous owners on this (or have the Realtor ask if they know of such drains), it would help. It would be good to know where 'it' comes and goes. It's not unusual to be on the crest of a 'hill' and be a party to an underground stream. Natural Pressure can push molten rock to the surface, ya know.
Let me know what you find out. You didn't say what the contractor's proposals were. Maybe you need 1 or 2 more to assess the situation. My best to ya and hope this helps.