new relationship energy? just a guess
On that premise I would say it is natural, and yes maybe some jealousy. Not the kind so much that you are jealous and want what someone else has but it perhaps brings you a nostalgia for your own NRE when it was happening?
Out of crisis comes opportunity! So they've got this energy, you might be wishing for a little bit of your own, so this is a good time to really appreciate and be grateful for what you have with R. A time to review and cherish the benefits you have had with this 2.5 years of smooth sailing, the progress you've made and if T's new relationship is showing you something that you want then there's no reason not to make the effort to find out exactly what it is that you desire and create an intention to achieve it!
Or if it's completely not jealousy at all, the dark cloud could be something else that has nothing to do with T or C's other relationships but rather your relationships with them. Or something more personal, just having to do with yourself. I know I have had some dark times that were very internal but then again at the same time I have never been impervious to external influence; many times a series of "straws on the camel's back" can lead us down a darker part of the path.