Shadow, since I'm not experienced with therapy I can't give you any professional advice. And I won't google the answer for you because I'm sure if you wanted a googled answer you would've sought it yourself.
BUT I will say that grief itself is natural. How long grief lasts is a different thing.
Can you try to pinpoint when exactly your feelings of grief about your pet are triggered? Is it when you watch something about animals on TV or is it when you're stressed? The fact that he died when in your arms might be a thing, too -- would you say you felt guilt about him dying? Can you describe the feelings you experience while you're grieving, the things you think about?
When I was little, my grandfather used to raise animals like pigs and goats and stuff. We kids played with the animals all the time -- I was particularly fond of the pigs. I loved them and found them to be very affectionate. The first time I found out they were being butchered was very traumatizing for me. My grandfather told me to lure one of them with a bowl of food. Told me to lure him to the back kitchen (which is like an outside kitchen). So of course "Casper" followed me and as soon as we got to the kitchen, my uncles locked me inside the house and told me not to peek. Well, I peeked and saw them kill the pig. The way they did it was horrible, the screams were horrible, and to this day I feel wrecked just thinking of it. For a long time I couldn't even eat pork.
With my feelings about these animals' deaths, I don't necessarily grieve them as one would for something that JUST died, but I would feel really bad about it and if I got to talking about it with someone I might even cry.
When you're thinking of your guinea pig, do you feel as if you lost him all over again or are your feelings more just sadness or are they maybe traumatic because you witnessed him die?
