Welcome to the site.
Well, I am sorry to learn about the situation that you find yourself in and hopefully you will come out if this.
Having said that I feel there is something brewing between the two of them which is beyond professional relations , mostly because of the discreet manner that the conversations are being conducted which happen almost always in your absence.
The fact that your husband has also been avoiding any sort of intimate liaison with you nowadays, adds
more fuel to the fire so as to put it.
I will strongly advise you to stop being a silent spectator to all of this that is going on beneath your nose and immediately confront your husband and seek explanation from him about the very nature of relationship that he has forged with this female colleague of his making him know that you suspect that it is much more than a professional discourse.
You should encourage him to open up about his feelings like if he finds lack of romance , passion or lack of attraction towards your so that you two can work together to add spark to your relationship and make it as fresh it was before. Also make him realise that your relationship has stood the test of time for nothing and together you can beat all odds including this hurdle where he finds himself getting attracted towards another female. Do not create a scene if he accepts his attraction towards this female because that may give him more reason to pursue a relationship with her, instead consider it as a problem which you together can beat by coming on the same page and bringing fresh romance to your life and relationship which could have become insipid and passionless after so many years.If you are going to get angry and snap at him if he accepts that he is drifting away then he may not take it well and it may give him more reason to move away from you but if you try to be patient and understanding and try to be forgiving then he may be more than keen on coming back to you all over again.Lastly, try to mention and discuss this in a firm manner but do not go out of control as far as your Anger is concerned. Try to give him enough time to explain his actions and do not be hasty in jumping to conclusions , see reason in his explanation and based on that plan your next move.I wish you all the best and I shall pray for you and hopefully things will become normal all over again.Please kindly leave a POSITIVE RATING if you are satisfied with the answer.Regards