DH had a month off work when DS arrived, so this is my first week on my own. All the housework is done and I have cooked dins every night. DH comes home and spends time with DS when he wakes up and baths him every day and cleans up the dinner dishes.
I feel exactly the same way you do - I feel so guilty whenever I sit down and do nothing! I am so used to working long hours at work and am not used to sitting down and watching TV or reading a book during the day, but what someone said was right - in paid work you are entitled to breaks!
You definitely need your DP's reassurance and I think it is a good idea that you let him know how you are feeling

I mentioned it to DH before he went back to work that it is going to take some adjusting for me and that I feel guilty and I have noticed he has made an effort to make me feel good this week. When I text him yesterday to say that DS was asleep in the afternoon (he hadn't been sleeping well during the day lately), he told me that that was great and that I could now go and relax for the afternoon. I ended up doing housework for a few hours, but it was nice that he suggested it!