What fourth amendment rights of yours do you believe were violated during that undercover investigation 3 months later and how is that related to the search/seizure that was done the day your husband died?Would it be worthwhile to try and sue the police for violating my 4th Amendment rights and for the damages they caused during their undercover investigation of me that started 3 months after the death of my husband?
If you have been charged with a crime and the search was done illegally then you certainly could move to suppress the evidence taken from the search so that it could not be used against you at trial. If you have not been charged with a crime and the investigation is closed you may sue to get the seized items returned to you (or your late husbands estate, if it's his stuff).
But you've not mentioned anything done during the undercover investigation that violated your rights or what damages that caused you, so I cannot comment on what remedies you may have for that.
If a criminal action is still pending or may be initiated you probably want to wait until that is resolved before filing a lawsuit. Discovery in a civil lawsuit is probably not something you want to be subject to while criminal charges are pending or a possibility. Consult a Colorado civil attorney about the possibility of pursuing a civil suit and the timing of any such suit.