Is it okay to be angry in this situation?

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Two months ago, there was this guy that I didn't even know who borrowed my laboratory coat. Since everyone in class was in a rush and I won't be needing it for the day, I lent it to him and told him that I'd meet him after his laboratory period so that I ca get my lab gown. What happened was, he failed to meet me. One week later, I didn't see him. Another week passed, I checked our college office class schedules to have their schedule so I could talk to him. When I got their schedule, I went to their section. Turns out that he was not around. So I told his classmates to tell him that I have been waiting for him to return it. Another week passed and I was starting to get annoyed. I tried to think positively that he can't escape me anyway. So one time, I waited for him in the laboratory. He was constantly apologizing because he didn't return it to me. I found out that he lost it in the research center. He said he'd replace it. After a month, I haven't received the replacement. He borrowed it from me before the prelims. Finals period is nearing. The semester's about to end. I got pissed off this morning because I had no lab gown to wear. I sent him an sms which says something like this: "Kuya" - literally "older brother" but a term for respect anyway for someone older than you Again. "Kuya, why is it taking you so long to replace my labgown. I easily lent you when you needed it and now you haven't replaced it. It's been a long time." So there. I tried not to be really rude but I can't help being a little rude. Just a few minutes ago he replied.. "Hey. I'll just leave the labgown to the technician since we don't meet. I'm sorry, I'm just really busy. Thank you." And here I am, kinda cooling down because of how he nicely replied and acted cool.. Which I think I shouldn't be feeling. I know I should be strict instead. Is it bad? But it is okay to be angry, right? I just thought of how my best friend would deal with this, she'd most prolly bombard the guy with text messages everyday or scare him by waiting for him in the laboratory and talk to him.